Supplements · Amino acids

Cysteine (L-Cysteine): Benefits, Foods, Dosage & Safety

What cysteine is, how your body turns it into glutathione, its master antioxidant, the foods richest in it, what the research shows for colds, blood sugar, your liver and hair, and how to take L-cysteine supplements.

By Dr. Josh Axe, DC, DNM, CNS·23-min read

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Evidence-based63 sources citedUpdated November 7, 2026

Published Nov 7, 2026Last updated Nov 7, 2026Editorial policyMedical review policyFact-checking policy

Cysteine supplements come as capsules, tablets and powders, sold as L-cysteine, L-cystine or NAC.

The Dr. Axe Score™

6.2/10

Overall Dr. Axe Score · Cysteine (L-Cysteine)

Cysteine scores 6.2 for its central job in building glutathione, its place as half of the glutathione pair in my Mitochondrial Longevity Protocol and a gentle safety profile, held back by a small number of human trials of the amino acid itself.

Scientific Evidence

5.2/10

Human trials of L-cysteine and L-cystine, from 7 people to 1,998 smokers (colds in 173 men, blood sugar in 165 adults, quitting smoking in two trials of 2,421 smokers, acetaldehyde in 7 to 19), plus two 2-week glutathione studies of cysteine with glycine; several were funded by product makers.

Bioavailability & Absorption

6.0/10

Cysteine from food protein, whey and NAC raised glutathione in people within 2 weeks to 3 months.

Clinical Effectiveness

6.0/10

Our judgment, counting the glutathione pair in the Mitochondrial Longevity Protocol and NAC in the bloodwork protocol: it restores glutathione when demand outruns supply, and trials point to fewer colds, lower acetaldehyde levels and steadier blood sugar.

Safety & Tolerability

8.2/10

A normal part of the protein you eat, with side effects at placebo rates in trials; people with cystinuria are the main exception, and interactions are rated separately.

Why this score?

Why Cysteine (L-Cysteine) scores 6.2 / 10

Weighting: Evidence 30% · Bioavailability & Absorption 25% · Clinical Effectiveness 25% · Safety & Tolerability 20% (5.2×0.30 + 6.0×0.25 + 6.0×0.25 + 8.2×0.20 = 1.56 + 1.50 + 1.50 + 1.64 = 6.20, shown as 6.2). How we score

A transparent, weighted index built for supplements. See how it’s calculated →

Interaction check: Moderate for supplements, Low for food. Cysteine is part of the protein you eat every day, and the few cautions below apply to supplements, NAC most of all.

Nitrates (for example, nitroglycerin). In a hospital trial, NAC strengthened the blood-pressure drop from nitroglycerin, so if you take a nitrate for chest pain, your cardiologist should approve NAC or any other cysteine supplement first.59 9

Blood thinners (for example, warfarin). On my show, I name blood thinners alongside nitroglycerin as the medications to be conscious of with NAC, so if you take one, check with your prescriber before adding NAC.9

Chemotherapy and radiation (for example, capecitabine). Some cancer treatments work by creating oxidative stress, so your oncology team decides whether and when you take antioxidant supplements, while cysteine from food stays on your plate.9

Activated charcoal and other binders. Binders trap whatever they meet, so take them at least two hours apart from cysteine supplements.60

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Cysteine at a glance

Cysteine is the sulfur-containing amino acid your body needs to make glutathione, its master antioxidant, and the building block in shortest supply whenever your cells need more of it.1 2

What it does for you: it rebuilds your glutathione when it has fallen behind and can mean fewer colds. In two 2-week studies, cysteine with glycine raised glutathione 94.6% in older adults and 64.4% in adults with type 2 diabetes, and in a placebo-controlled winter trial of 173 men, 11.4% caught a cold on cystine and theanine, against 27.1% on placebo3 4 5
Where to get it: chicken (about 337 mg in 3 ounces), turkey, wild-caught salmon, eggs (about 136 mg in a large egg), seeds and legumes6 7
How much: adults need about 1,330 mg a day of methionine and cysteine combined at 154 pounds; as a supplement, I recommend NAC at 600 to 2,400 mg a day, and L-cysteine trials used 300 to 1,200 mg8 9 10 11
How to use it: put protein on your plate at every meal with sulfur-rich vegetables beside it, pair cysteine with glycine and the nutrients that help make glutathione, and take supplements with food12 9 5

Cysteine is a sulfur-containing amino acid your body uses to make glutathione, its master antioxidant, and it's the building block in shortest supply whenever your cells need to make more.1 2 My approach to it is food first, with eggs, wild-caught fish, poultry, seeds and legumes supplying it every day, and when your glutathione needs extra support, I recommend NAC, the supplement form of cysteine, at 600 to 2,400 milligrams a day for most people.9

Your body was designed to make its own glutathione around the clock. Age, alcohol, everyday pain relievers like acetaminophen, high blood sugar and the toxins you breathe and eat all push the demand higher, and when they do, cysteine is the ingredient your cells run short of first.3 4 2 9 So when your glutathione falls behind, the root cause lies in what's draining it and what's missing from your plate, and you can change both. When I work with patients, the goal is to teach the body to create glutathione on its own, and cysteine is the raw material that makes that possible.9

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Did you know?

Three tablespoons of hulled hemp seeds carry about 202 mg of cystine, close to the 232 mg in 3 ounces of wild salmon.13 14
USDA's food tables list cysteine as cystine, two cysteine molecules joined at their sulfur atoms, so cystine is the number you'll see in the food table below.7 15
The FDA allows L-cysteine in yeast breads as a dough strengthener, at no more than 0.009 part for every 100 parts of flour.16

What is cysteine? L-cysteine, cystine and NAC explained

Quick answer

Cysteine is one of the amino acids that make up protein, and with methionine, it's one of the two that carry sulfur.15 8 L-cysteine is the form found in food and in your body, and because your body can also make it from methionine, it's called conditionally essential: you usually make enough, but your need for it climbs under stress and illness.2 15

The sulfur is what sets cysteine apart. It sits on cysteine's side chain as a reactive group, and that group lets cysteine link up with a partner, grab onto harmful compounds and give glutathione its protective power.15 1 17 When your body is under strain from illness or inflammation, its need for cysteine rises, because cysteine is the scarce ingredient for making more glutathione.2

Your liver can make cysteine from methionine, the other sulfur amino acid, through a pathway that passes through homocysteine.15 B vitamins keep that pathway moving, and on my show I've named the ones it depends on: methylfolate, a methylated form of B12, vitamin B6 and B2, along with folate foods such as leafy greens and beans. Homocysteine on your blood work shows how well that cycle is running.18

You'll see cysteine sold three ways, and the names are close enough to confuse anyone:

L-cysteine

What it is: the amino acid itself, with its sulfur group free to work. How it's studied: 300 to 1,200 mg a day for stomach comfort, blood sugar and hangovers.10 19 11

L-cystine

What it is: two cysteine molecules joined at their sulfur atoms.15 How it's studied: 700 mg a day with theanine for colds, and in combination formulas for hair.5 20

NAC (N-acetylcysteine)

What it is: cysteine with an acetyl group attached, which your body removes to free the cysteine. How it's used: a hospital drug and a widely sold supplement for raising glutathione.21 1

Most of the trials on this page, for colds, hair, stomach comfort, hangovers and blood sugar with vitamin D, used L-cysteine or L-cystine. The glutathione studies gave cysteine as NAC, the same form behind the lung, liver and mood research and the one hospitals use as a drug. Our guide to NAC (N-acetylcysteine) covers that research in depth. As I explain on my show, NAC works by handing your cells cysteine so they can build glutathione on their own.9

How does cysteine work in your body?

Quick answer

Cysteine carries the sulfur that does glutathione's work, and the amount of cysteine your cells have on hand, together with the enzyme that starts the job, sets how fast glutathione gets made.1 Its sulfur can also bind acetaldehyde, a toxic by-product of alcohol that's also in cigarette smoke.22 23

Cut an apple, leave it on the counter, and within minutes the flesh turns brown. That's oxidation, the same chemistry that rusts metal, and a version of it runs inside your cells all day long.9 Glutathione, which your cells make for themselves, is one of their main defenses against it, and I like to think of it as the skin of the apple, a living antioxidant coat that protects every cell.9 It's built from three amino acids on a kind of cellular assembly line: glutamate, cysteine and glycine.1 Most diets supply plenty of glutamate, so the pace is set by the other two, and cysteine most of all.1 9

Cysteine is what allows your body to get through the bottleneck in making glutathione, and it carries the sulfur that does the actual work of putting glutathione together.Dr. Josh Axe, DC, DNM, CNSFrom Your Body Makes Its Own Master Antioxidant — Here's Why It Stops, The Dr. Josh Axe Show (2026), at 6:46

That assembly line slows with age. In a Baylor College of Medicine study comparing 8 older and 8 younger adults, the older group had less cysteine in their red blood cells (19.8 against 26.2 µmol/L) and a little more than half the glutathione (1.12 against 2.08 mmol/L), and they made it far more slowly.3 Uncontrolled blood sugar slows it too: the same research team found glutathione production running well behind in adults with uncontrolled type 2 diabetes.4

Daily life adds its own drains, and as I've said on my show, alcohol is the single biggest one, followed by acetaminophen and many other medications, while sleep debt, smoking and vaping, overtraining without recovery, heavy metals and chronic infections all add to the load.9 Each one uses up glutathione faster, and each time, your cells reach for more cysteine to replace it.

Here is what your cells do with the cysteine they get:

1You eat it or make it

Protein foods bring cysteine to your plate, and your liver can make more from methionine by way of homocysteine.6 15

2It sets the pace

The amount of cysteine on hand, along with the enzyme that starts the job, decides how quickly glutathione gets made.1

3Two links join the chain

Cysteine joins glutamate first, then glycine is added, and the finished glutathione carries cysteine's sulfur as its working end.1

4Glutathione goes to work

It neutralizes oxidants before they damage your cells, and in your liver it binds toxins and escorts them out of your body.1

5Your levels climb back

In people whose glutathione had fallen behind, 2 weeks of cysteine with glycine brought it back up, as the first benefit below shows.3 4

Cysteine has a second job that doesn't run through glutathione. Its sulfur group can bind acetaldehyde, the toxic by-product your body makes from alcohol and the same compound found in cigarette smoke, which is the idea behind the slow-release tablets and lozenges in the fifth benefit below.22 23

Cysteine benefits: what is cysteine good for?

Quick answer

Cysteine is best supported for raising glutathione and supporting your liver's detox work, and early human trials also point to fewer colds, steadier blood sugar, lower acetaldehyde levels from alcohol and smoke, and fuller hair.3 4 21 5 19 22 20

We've ranked these six benefits by the human research behind each one, beginning with the job cysteine does in every cell, and NAC's own results for your lungs and mood come after them.

Builds your glutathione

Moderate evidenceHuman studyStudies of 8 older adults and 12 adults with type 2 diabetes before and after 2 weeks, no placebo group; placebo-controlled trial · 24 older adults; whey trials · 36 and 21 adults

When your glutathione has fallen behind, giving your body more cysteine along with glycine brings it back, and the change shows up within weeks. After 2 weeks of cysteine and glycine in that Baylor study, with no placebo group, the older adults' glutathione was 94.6% higher and they were making it 78.8% faster, with less oxidative stress, until they no longer differed from the younger group.3 The same team gave cysteine (as NAC) with glycine for 14 days to 12 adults with uncontrolled type 2 diabetes, and the rate at which their red blood cells made glutathione rose 85.1%, while glutathione itself rose 64.4%, from 1.65 to 2.72 µmol per gram of hemoglobin.4

A 16-week placebo-controlled trial of 24 older adults found that the same pairing, called GlyNAC, given at 100 mg per kilogram of body weight of each a day, corrected glutathione deficiency and improved oxidative stress, insulin resistance, walking speed and strength: red-cell glutathione rose from 0.4 to 1.4 mmol/L, the same as in young adults, and walking speed from 1.13 to 1.34 meters per second, while both stayed about where they started on placebo.24 Food protein rich in cysteine does this too. In a 2-week study of 36 healthy adults (31 finished) with no placebo group, 45 grams a day of a pressurized whey protein isolate raised glutathione in white blood cells by about 24%, while 15 grams didn't, and in a 3-month placebo-controlled trial of 21 young adults with cystic fibrosis, 20 grams of whey a day raised it 46.6%.25 26

Why does that matter to you? In a study of 33 older adults living at home, those with more glutathione in their blood had fewer illnesses and rated their own health higher, and glutathione alone accounted for 24% of the differences in their illness scores.27

Study takeaway: in two 2-week studies, cysteine with glycine raised glutathione 94.6% in older adults and 64.4% in adults with type 2 diabetes.

Two weeks of cysteine and glycine raised glutathione 64.4%

Source: Sekhar et al. · Diabetes Care, 2011 [4]Glutathione in red blood cells before and after 14 days of cysteine and glycine.

Liver and detox support

Moderate evidenceHuman studyU.S. study of 2,540 people treated with NAC after an acetaminophen overdose, no placebo group; trial of 171 lead-exposed workers with an untreated comparison group (NAC)

Your liver clears toxins in two phases, and in the second, it attaches compounds such as glutathione so the toxin can leave your body, which is why cysteine supply matters so much there.1 28 As I've said, "Cysteine is really important for liver function."28

The clearest example comes from the hospital. When an overdose of acetaminophen burns through the liver's glutathione, the antidote doctors give, at hospital doses, is NAC, the supplement form of cysteine, and in a U.S. study of 2,540 people treated after an overdose, with no placebo group, liver injury developed in 6.1% of those at risk when NAC began within 10 hours, against 26.4% when it began 10 to 24 hours later.21

Cysteine's sulfur also binds heavy metals.17 In Poland, 171 men exposed to lead at work were split at random into no supplement or NAC at 200, 400 or 800 mg a day for 12 weeks, and blood lead fell from where it started in all three NAC groups, while red-cell glutathione rose 5% at 400 mg and 6% at 800 mg.29 Our guides to liver function and heavy metal detox cover the rest of that plan, and day to day, the protein and sulfur-rich foods below keep your liver supplied.

Study takeaway: hospitals give NAC, the supplement form of cysteine, as the antidote when acetaminophen burns through the liver's glutathione.

Fewer colds and stronger immune defenses

Emerging evidenceHuman studyPlacebo-controlled trials · 173 men (colds), 65 nursing-home residents (flu shot), 16 runners and 32 surgery patients

Cystine, taken with theanine, has helped healthy men get through cold season with fewer colds. In a trial run in Japan from January to February, 173 healthy men took 700 mg of cystine with 280 mg of theanine a day, or a glycine placebo, for 35 days, and neither the people nor the researchers knew who got what. Only 11.4% of the cystine group caught a cold, against 27.1% on placebo, and fewer had a fever (10.2% against 28.2%) or chills (11.4% against 24.7%), though colds lasted about as long in both groups, in a trial the tablets' maker funded and ran.5

Smaller placebo-controlled trials have tested the same pairing where immune defenses are under strain, and researchers think it works by raising glutathione and calming the inflammatory response.30 In 65 nursing-home residents who took it for 2 weeks before a flu shot, 63% of those with low blood protein responded to the H1N1 strain, against 10% on placebo, and across the whole group, the shot raised antibody levels about equally in both groups.31 In 16 long-distance runners, it eased the spike in inflammatory white blood cells after hard training on the first day of a camp,32 and in 32 people having surgery on the esophagus, the cystine group walked farther in 6 minutes a week after surgery and scored better on recovery at day 13, though recovery of daily activity, the main measure, came out about the same.30

If you'd like to follow that winter trial, the how-to section below has its amounts and timing.

Study takeaway: in a placebo-controlled winter trial of 173 men, 11.4% caught a cold on cystine and theanine, against 27.1% on placebo.

Fewer colds on cystine and theanine: 11.4%, against 27.1% on placebo

Source: Kurihara et al. · Journal of Amino Acids, 2010 [5]Share of men who caught a cold during the 35-day trial; lower is better.

Blood sugar support

Emerging evidenceHuman studyPlacebo-controlled trial · 165 vitamin D-deficient adults, 96 finished; pilot randomized trial · 35 adults with type 2 diabetes

Steadier blood sugar and a healthy glutathione supply go together, and in adults with uncontrolled diabetes, restoring glutathione was the point of the 14-day study in the first benefit.4 Two trials have now measured blood sugar itself. In a 6-month placebo-controlled trial of 165 African American adults low in vitamin D, those who took 1,000 mg of L-cysteine a day together with 2,000 IU of vitamin D ended with lower HbA1c and HOMA-IR, a measure of insulin resistance, than those who took vitamin D alone. In the men, CRP, a marker of inflammation, fell from where it started, while the drop in HbA1c and HOMA-IR was clearest in women. Of the 165, 96 finished the trial.19

In a 14-week pilot trial of 35 adults with type 2 diabetes, a cysteine-rich protein made from keratin, taken with whey, raised insulin-driven glucose clearance 26% compared with the control group, while whey alone made no difference.33 For keeping your numbers in the normal blood sugar range, the food habits in the how-to section are the place to start, with protein at every meal.

Study takeaway: in a 6-month placebo-controlled trial, L-cysteine with vitamin D lowered HbA1c and insulin resistance more than vitamin D alone.

Helps clear acetaldehyde from alcohol and smoke

Emerging evidenceHuman studyStudies in which each person tried both · 9 men and 7 adults with atrophic gastritis; placebo-controlled trial · 19 men; 2-year study · 127 people, not randomized; pooled analysis of 2 trials · 2,421 smokers

When you drink, your body turns alcohol into acetaldehyde, a toxic by-product that's also in cigarette smoke, and cysteine's sulfur binds it directly.22 23 In a study in which 9 healthy men each tried both a slow-release L-cysteine tablet and a placebo tablet, held under the upper lip during a drinking session, acetaldehyde in their saliva was 59% lower with the cysteine.22

The same idea helps people with atrophic gastritis, a condition in which the stomach makes little acid and bacteria in it turn alcohol into acetaldehyde. In 7 adults who each tried both, 200 mg of slow-release L-cysteine left acetaldehyde in the stomach at 4.7 µM, against 13 µM on placebo.34 And over 2 years, 77 people with atrophic gastritis who took 300 mg a day saw their symptom score fall from 7.8 to 4.5, while 50 untreated people saw no change, in a study that wasn't randomized.10

For the morning after, which our guide on how to get rid of a hangover covers in full, a placebo-controlled trial of 19 healthy men found that a vitamin supplement providing 1,200 mg of L-cysteine eased hangover, nausea and headache, and 600 mg eased stress and anxiety, by the researchers' analysis, which other researchers have questioned.11 35 For smokers working on quitting smoking, a pooled analysis of 2 placebo-controlled trials with 2,421 smokers, written by researchers tied to the lozenge's maker, found 29% higher odds of quitting with a slow-release L-cysteine lozenge.36

The simplest way to have less acetaldehyde to clear is to drink less of what makes it, and my recommendation is either no alcohol or 1 to 2 drinks a week at most.37

Study takeaway: slow-release L-cysteine lowered acetaldehyde in saliva by 59% after drinking, in a study in which 9 men each tried both.

Healthier, fuller hair

Emerging evidenceHuman studyPlacebo-controlled trials of L-cystine combinations · 47 adults (with gelatin and vitamin A) and 80 adults (with saw palmetto and other plant extracts)

Keratin, one of the main proteins in your hair and skin, is packed with cysteine, and the sulfur bridges between those cysteine units give each strand its strength.15 In a placebo-controlled German trial of 47 people with diffuse hair loss, a daily combination of 70 mg of L-cystine with gelatin and vitamin A cut the share of resting (telogen) hairs by 13.5%, while nothing changed on placebo.38 In a 6-month placebo-controlled trial of 80 adults with long-term shedding or pattern hair loss, a supplement pairing L-cystine with saw palmetto, pumpkin seed and pygeum extracts raised hair density by 12.3 hairs per square centimeter, against 1.5 on placebo, and several of its authors work for the product's maker.20

Both trials tested combinations, and a study of 30 women with long-term shedding found their cysteine levels weren't low.39 Our guides to vitamins for hair growth and hair loss remedies cover the rest of the plan, and since hair is built from what you eat, steady protein from the cysteine-rich foods below is where it starts.

Study takeaway: in a 6-month placebo-controlled trial of 80 adults, an L-cystine combination raised hair density by 12.3 hairs per square centimeter, against 1.5 on placebo.

What NAC adds: lungs, mood and fertility

NAC, the cysteine supplement I recommend for most people, also supports your lungs and your brain, and our NAC guide covers that research in full.9 A Cochrane review of mucus-thinning medicines, NAC and the related carbocysteine among them, found that people with chronic bronchitis or COPD had fewer flare-ups on them, with about 8 people treated for nine months keeping one more person free of flare-ups.40 NAC also steadies glutamate, a chemical messenger in the brain, which is why researchers have tested it for compulsive habits, cravings and low mood, and its research extends to ovulation in women with PCOS and to semen quality in men after varicocele surgery.41 42 43

Which foods are highest in cysteine?

Quick answer

Poultry, fish, eggs, seeds and legumes are the richest everyday sources: 3 ounces of roasted chicken breast carries about 337 mg of cystine, a cup of cooked lentils about 234 mg and a large egg about 136 mg.6 44 7 USDA measures cysteine in foods as cystine, so that's the figure in the table below.

Food is where I want you to start, because the protein on your plate delivers cysteine along with the glycine, B vitamins and minerals your body uses with it. These are the best everyday sources, from the high-protein foods you probably already eat.

Calculated from USDA FoodData Central (SR Legacy) at each record's own serving weight; 3 ounces is 85 grams.

Food	Serving	Cystine per serving
Chicken breast, roasted	3 oz (85 g)	about 337 mg6
Sardines, canned in oil, with bone	1 can (92 g)	about 243 mg45
Turkey, light meat, roasted	3 oz (85 g)	about 241 mg46
Lentils, cooked	1 cup (198 g)	about 234 mg44
Wild Atlantic salmon, cooked	3 oz (85 g)	about 232 mg14
Hemp seeds, hulled	3 tablespoons (30 g)	about 202 mg13
Chickpeas, cooked	1 cup (164 g)	about 195 mg47
Egg, whole	1 large (50 g)	about 136 mg7
Sunflower seeds, kernels	1 oz (28 g)	about 128 mg48
Goat milk	1 cup (244 g)	about 112 mg49
Pumpkin seeds, kernels	1 oz (28 g)	about 94 mg50
Brazil nuts	1 oz, about 6 nuts	about 87 mg51
Sheep milk	1 cup (245 g)	about 86 mg52
Spirulina, dried	1 tablespoon (7 g)	about 46 mg53

Plant foods do well here, too: hemp seeds rival fish by the spoonful, and a cup of lentils or chickpeas adds about 195 to 234 mg, which helps if you eat less meat.13 44 47 Oats are a good grain source too, at about 408 mg per 100 grams dry.54 For dairy, goat milk leads, at about 112 mg a cup, with sheep milk close behind.49 52

With eggs, choose pasture-raised and eat the yolk too.18 If you use a protein powder, whey protein is a concentrated source of cysteine, and I like it organic, A2 and grass-fed; if whey doesn't suit you, a plant or egg protein works as well.2 55

Then there are the foods that help your body put cysteine to work. When my mom was battling cancer, I was very conscious of what we had her eat constantly: broccoli sprouts first, then cruciferous vegetables such as kale, cabbage and Brussels sprouts, then garlic and onions from the same sulfur family, along with eggs, clean protein like salmon, and Brazil nuts for their selenium.9 Garlic and onions bring sulfur compounds of their own that help your body make glutathione, so they belong beside your protein at every meal. Our list of sulfur-rich foods has more ideas, and here's how I put it in my game plan for glutathione:

Add sulfur to your meals with cruciferous vegetables, garlic, onions, eggs, sauerkraut and sprouts. Those are fantastic.Dr. Josh Axe, DC, DNM, CNSFrom Your Body Makes Its Own Master Antioxidant — Here's Why It Stops, The Dr. Josh Axe Show (2026), at 35:07

How to use cysteine: food first, then L-cysteine supplements

Quick answer

Start with protein at every meal and sulfur-rich vegetables beside it, and if you want more support, add NAC or L-cysteine with food, paired with glycine and the vitamins and minerals your body uses to make glutathione.12 9 For NAC, I recommend 600 to 2,400 mg a day for most people.9

As I put it on my show, "The healthiest cell isn't the one with the most glutathione. It's the one making and recycling the most glutathione."9 I sum up the plan for getting there in three steps: build the factory, close the drain and check the gauge.9 Cysteine belongs to the first step, and here's how to put it to work.

Six easy ways to add cysteine to your day:

Eggs at breakfast. Pasture-raised eggs, yolk and all, or a breakfast salmon egg bake that brings two top sources together.

A lentil or chickpea lunch. A bowl of red lentil soup or a scoop of homemade hummus adds plant cysteine.

Poultry at dinner. A pot of turkey chili covers several meals, and roasted chicken breast leads the food table.

Seeds on top. Three tablespoons of hemp seeds on a salad or in a smoothie add about 202 mg, and sunflower or pumpkin seeds work too.

Sulfur on the side. Broccoli sprouts, cabbage, garlic, onions and sauerkraut bring the sulfur compounds that help your body make glutathione.

A glass of goat milk. A cup carries about 112 mg, and it blends well with berries and hemp seeds.

How much cysteine do you need?

Adults need about 19 mg per kilogram of body weight a day of methionine and cysteine combined, which works out to about 1,330 mg for someone who weighs 154 pounds.8 In U.S. survey data from 1988 to 1994, people averaged about 1,000 mg of cysteine a day from food and supplements,5 8 and in national survey data from 2011 to 2018, fewer than 1% of adults fell below the average requirement for methionine and cysteine.56 So when glutathione runs low, look first at what's raising the demand: age, alcohol, medications and blood sugar strain.3 4 9

My protein guideline is 30 to 40 grams at each of three meals, aiming closer to 1 gram per pound of body weight a day once you're over 40.12 55

L-cysteine supplement forms and dosage

If you want more than food provides, these are the forms and the daily amounts used in the research and in my protocols:

Form	Daily amount	How it was used
NAC	600 to 2,400 mg	My range for most people; 600 to 1,200 mg in my bloodwork protocol when GGT runs high9 57
L-cysteine	300 mg	Slow-release, for 2 years, in people with atrophic gastritis10
L-cysteine	1,000 mg	With 2,000 IU of vitamin D, for 6 months19
L-cysteine	600 or 1,200 mg	In a vitamin supplement, for one night of drinking11
L-cystine	700 mg	With 280 mg of theanine, for 35 days5
Whey protein isolate	20 to 45 g	For 2 weeks to 3 months, to raise glutathione26 25

NAC, ideally paired with glycine as GlyNAC, is the form I recommend for most people, and I personally take around 1,200 to 1,800 mg of NAC a day.9 28 Here's what I've found in patients: a dose well below the trial amounts still works, and once your body has enough stored up, you can bring the dose down. Just don't go too low, because with GlyNAC, an underdose is one of the mistakes I see people make, and doses of at least 1,000 mg each of glycine and NAC typically get the best results.9 L-cysteine and L-cystine make sense when you're following one of the specific trials above, such as cystine with theanine for the cold season.5

Some people need more because a variant in one of the detox genes leaves them less able to clear mold, and for my patients who come in with this genetic variant, taking a lot of N-acetylcysteine and liposomal glutathione, along with glutathione IVs, changes their lives.18

Pair it with its partners

Cysteine does its best work alongside glycine, its partner in glutathione, which runs short with age as well: in the Baylor study, the older adults had 218.0 µmol/L of glycine in their red blood cells, against 486.7 in the younger group.3 I recommend taking about 3 grams of glycine at night, since it supports phase two liver detoxification,28 and in my Mitochondrial Longevity Protocol, glycine and cysteine, given as NAC, are the glutathione pair you replenish, because glutathione levels fall with age.58

Your body also needs selenium, vitamin B2, magnesium and vitamin C to make and recycle glutathione.9 I saw how much those partners matter with a patient who had struggled with severe rosacea for years. Her GGT, a liver enzyme on her blood work, was in the normal range but not optimal, and she was also low in vitamin B2, selenium and magnesium, all required for glutathione recycling. So along with N-acetylcysteine plus glycine and glutathione-building foods, we brought those nutrients up too, and her skin cleared by about 98% in 8 weeks.9

It's not just about glutathione by itself. It's about making sure your body has all the things it needs for running its glutathione factory.Dr. Josh Axe, DC, DNM, CNSFrom Your Body Makes Its Own Master Antioxidant — Here's Why It Stops, The Dr. Josh Axe Show (2026), at 9:10

The third step is checking the gauge, and GGT is the marker to ask for at your next blood draw. It doesn't measure glutathione itself, but when it climbs, it tells you oxidative stress is rising and your body needs more glutathione.9 When GGT runs high, my bloodwork markers protocol uses 600 to 1,200 mg of NAC to support glutathione.57

When to take it and how long

Take cysteine supplements with meals. In the winter cold trial, people took their tablets after breakfast and after dinner, and food also helps if your stomach is sensitive.5 The studies on this page ran from 2 weeks for glutathione to 35 days for colds, 6 months for blood sugar and hair, and 2 years for stomach comfort, so give it at least several weeks and keep it as a daily habit while you work on the drains.3 4 5 19 20 10

Is cysteine safe? Side effects and who should be careful

Quick answer

Yes, cysteine is safe for most people: it's part of the protein you eat every day, and in a 35-day placebo-controlled trial of 173 men, stomach complaints such as nausea and diarrhea were about as common on cystine and theanine as on placebo.5 A 6-month placebo-controlled trial of an L-cystine combination in 80 adults found it well tolerated.20

Before you start a cysteine supplement:

Stomach upset. Nausea or an upset stomach can happen at supplement doses, so take cysteine with food and split larger amounts into two doses.5
Cystinuria. This inherited condition lets cystine build up in the urine and form kidney stones, so if you have it, skip cysteine and cystine supplements and follow your kidney team's plan for fluids and diet.61 62
Pregnancy and breastfeeding. Food sources are fine; as with almost every supplement, check with your physician before taking a cysteine supplement while you're pregnant or nursing.9
Acetaminophen overdose. This is a hospital emergency, so get emergency care right away, even if you feel fine; NAC is given there as the antidote.21

The bottom line

Should you take cysteine?

Cysteine is the sulfur-carrying building block your body needs to make glutathione, and for most people, the best place to get it is the table: eggs, poultry, wild-caught fish, seeds and legumes, with sulfur-rich vegetables beside them. The research is strongest for restoring glutathione when it has fallen behind and for supporting your liver, and early trials point to fewer colds, steadier blood sugar, lower acetaldehyde levels and fuller hair.

If you want more support, NAC at 600 to 2,400 mg a day is the form I recommend for most people, taken with food and paired with glycine and the nutrients that keep the glutathione factory running. Your body is repairing, recycling and rebuilding every second of the day, and our job is to remove what interferes with that design and give it the raw materials it needs.

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Cysteine: frequently asked questions

What is L-cysteine used for?

L-cysteine is used to help your body make glutathione, its master antioxidant, and in human trials it has been studied for colds, blood sugar, clearing acetaldehyde after drinking and hair loss.1 5 19 22 20 In a placebo-controlled trial of 173 men, cystine with theanine cut colds to 11.4%, against 27.1% on placebo, and in a 6-month placebo-controlled trial of 165 adults (96 finished), L-cysteine with vitamin D lowered HbA1c more than vitamin D alone.5 19 It's also used as a dough strengthener in bread.16

What foods are high in cysteine?

Poultry, fish, eggs, seeds and legumes are the richest sources. By USDA figures, 3 ounces of roasted chicken breast carries about 337 mg of cystine, a can of sardines about 243 mg, a cup of cooked lentils about 234 mg, 3 ounces of wild salmon about 232 mg, three tablespoons of hemp seeds about 202 mg and a large egg about 136 mg.6 45 44 14 13 7

Is cysteine the same as cystine?

No. Cystine is two cysteine molecules joined at their sulfur atoms.15 USDA measures the cysteine in foods as cystine, and cystine is the form used in the placebo-controlled cold trial of 173 men, at 700 mg a day with theanine.7 5

Is NAC the same as L-cysteine?

No. NAC, or N-acetylcysteine, is cysteine with an acetyl group attached, and your body removes that group to free the cysteine for making glutathione.1 9 NAC is the form hospitals give as the antidote for acetaminophen overdose, and it's the form Dr. Axe recommends for most people, at 600 to 2,400 mg a day.21 9

What are the symptoms of low cysteine?

Low cysteine usually shows up as low glutathione rather than as a set of symptoms of its own. In a Baylor College of Medicine study comparing 8 older and 8 younger adults, the older group had less cysteine in their red blood cells (19.8 against 26.2 µmol/L) and a little more than half the glutathione, and age, alcohol, medications and blood sugar strain all raise the demand.3 4 9 Most adults who eat enough protein meet the basic requirement (fewer than 1% fell below it in U.S. survey data from 2011 to 2018), so GGT on a standard blood panel is the practical way to see whether your glutathione is keeping up.56 9

Is cysteine good for your liver?

Yes. Your liver uses cysteine to make glutathione, which it attaches to toxins so they can leave your body, and Dr. Axe calls cysteine really important for liver function.1 28 Hospitals give NAC, the supplement form of cysteine, as the antidote for acetaminophen overdose, and in a U.S. study of 2,540 people with no placebo group, liver injury developed in 6.1% of those at risk when NAC began within 10 hours.21

Can I take L-cysteine every day?

Yes, in the amounts studied. A placebo-controlled trial of 165 adults (96 finished) gave 1,000 mg of L-cysteine a day for 6 months, and a 2-year study of 77 people with atrophic gastritis, with no placebo group, gave 300 mg a day.19 10 Take it with meals to keep your stomach comfortable.5

Is L-cysteine made from hair?

Some of it has been. L-cysteine for the food and supplement industries has traditionally been extracted from animal hair and feathers, which are rich in the cysteine-packed protein keratin, and fermentation by microbes is the newer alternative.63 15 If the source matters to you, choose a supplement whose label says it's made by fermentation.

Is there L-cysteine in bread?

There can be. The FDA allows L-cysteine as a dough strengthener in yeast breads, at no more than 0.009 part for every 100 parts of flour.16

Does cysteine help hair growth?

It may, as part of a combination. In a 6-month placebo-controlled trial of 80 adults with long-term shedding or pattern hair loss, a supplement pairing L-cystine with saw palmetto and other plant extracts raised hair density by 12.3 hairs per square centimeter, against 1.5 on placebo.20 Keratin, the main protein in hair, is rich in cysteine.15

What are the side effects of L-cysteine?

Mild stomach upset is the main one, and in a 35-day placebo-controlled trial of 173 men, nausea, diarrhea and other stomach complaints were about as common on cystine and theanine as on placebo.5 Take it with food to keep your stomach comfortable, and skip supplements if you have cystinuria, an inherited condition that forms cystine kidney stones.61

Does L-cysteine help with hangovers?

It may. In a placebo-controlled trial of 19 healthy men, a vitamin supplement providing 1,200 mg of L-cysteine eased hangover, nausea and headache.11 Cysteine also binds acetaldehyde, the toxic by-product of alcohol, and Dr. Axe recommends either no alcohol or 1 to 2 drinks a week at most.22 37

Should I take L-cysteine or NAC?

For most people, NAC, ideally paired with glycine: it's the form Dr. Axe recommends, at 600 to 2,400 mg a day, and he takes around 1,200 to 1,800 mg of NAC a day himself.9 28 L-cysteine and L-cystine fit when you're following a specific trial, such as 700 mg of cystine with 280 mg of theanine a day, which cut colds in a placebo-controlled trial of 173 men.5

Freshness & update log

November 7, 2026 — Rewrote "L-Cysteine: Antioxidant Amino Acid that Boosts Lung and Brain Function," which had no references, under Dr. Josh Axe's byline. The page now leads with "cysteine," explains how cysteine becomes glutathione with a step-by-step diagram, sets out the differences among L-cysteine, L-cystine and NAC, ranks six benefits with each study's design and size, and adds two charts, a USDA food table, doses as studied, Dr. Axe's NAC range and food-first plan, one safety section with an interaction check and 63 references. It keeps the old page's foods and supplements-and-dosage sections and its antioxidant, detox, blood sugar, digestive, lung and brain topics, with lung, brain and fertility results labeled as NAC research. It corrects the claims that NAC is another name for L-cysteine and a form of L-cystine, that cysteine is the limit for making "glutamine" (it's glutathione), that glutathione predicts lifespan, that NAC improved sperm concentration, that the cited diabetes study showed balanced blood sugar, and that cruciferous vegetables supply cysteine. It removes pork from the food lists, the ulcerative colitis claim, the unsourced "more than seven grams" toxicity line, the kidney-function caution, the immunosuppressant and antifungal cautions, the wheezing and allergy claims and the list of preliminary uses for acne, angina, asthma, emphysema and cancer, none of which had human cysteine evidence behind them.

References

Dr. Josh Axe, DC, DNM, CNS

Doctor of Natural Medicine · Founder, DrAxe.com

Dr. Axe is a certified doctor of natural medicine, clinical nutritionist, and multiple New York Times bestselling author. He has spent 20+ years in clinical practice helping people heal at the root cause and founded one of the most-visited natural-health platforms in the world.

Read the full bio →

This article is for educational purposes. See our medical disclaimer.

On this page

What is it?
How it works
Benefits, ranked
Foods
How to use
Safety
The bottom line
Should you take cysteine?
FAQ

Quick facts

What it isA sulfur-containing amino acid and the scarce building block of glutathione, your body's master antioxidant
Top food sourcesChicken, turkey, wild-caught salmon, sardines, eggs, hemp and sunflower seeds, lentils and chickpeas
Daily needAbout 19 mg per kg of body weight of methionine and cysteine combined, about 1,330 mg at 154 pounds
Dr. Axe's supplement rangeNAC, the supplement form of cysteine, at 600 to 2,400 mg a day for most people
Best paired withGlycine, plus selenium, vitamin B2, magnesium and vitamin C
Interaction checkModerate for NAC and other supplements, Low for food; nitrates, blood thinners, chemotherapy or radiation, and binders such as activated charcoal

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