What Is L-Cysteine?

L-cysteine is an amino acid found naturally in the human body. It is one of the amino acids that are building blocks of the powerful antioxidant glutathione. It also is found in many protein-rich foods and sold as a dietary supplement.

There is some evidence for the benefits of L-cysteine supplements. It may help ease flu symptoms. It also may help treat some inflammatory diseases, and help diabetics to manage their illness. This article looks at the research behind possible uses, although they remain uncertain.

Milk eggs and legumes on a table have l-cysteine in them
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What Is L-Cysteine Used For?

L-cysteine may be used as a natural treatment for conditions that include:

L-cysteine also may support lung health in people with chronic obstructive pulmonary disease (COPD), help prevent colon cancer, boost sports performance in athletes, and promote detox.

But despite the many claims about L-cysteine, there still isn't a wealth of knowledge about the effects of using it. A 2018 review published in the journal Molecules noted the amino acid's benefits are unclear and further research is needed.


L-cysteine is found in many foods we eat. Pork chops, beef, chicken, and tuna are all good sources. So are oatmeal, eggs, and yogurt. Adding more to your diet through pills and powders may bring health benefits, but scientists are still studying the evidence for using L-cysteine.


Research suggests that L-cysteine may help when treating diabetes because it lowers blood sugar, reduces insulin resistance, and limits blood vessel damage. This may be especially important for Black people who have higher rates of diabetes, heart disease, and a genetic type of enzyme deficiency seen more often in men. It's called G6PD for short and linked to many complications.

A 2018 study published in the journal Diabetes found that L-cysteine helps to offset G6PD deficiency. High blood sugar counts can contribute to low G6PD levels, and related blood and blood vessel problems, but L-cysteine appears to restore those levels and improve function.

An earlier study published in the Canadian Journal of Physiology and Pharmacology found that cysteine-rich whey protein improves glucose metabolism in people and animals with type 2 diabetes. However, the study authors noted that more research is needed.

Free Radicals

L-cysteine may help prevent the exercise-related production of free radicals, which is shown to contribute to oxidative stress. It acts as an antioxidant.

However, as the Molecules authors noted, most of the clinical trials used to test L-cysteine have focused on the effects of a related derivative called N-acetyl-l-cysteine (NAC). There is even more recent interest in how NAC might be used to help treat people with COVID-19.

It's important to know that NAC is not approved by the Food & Drug Administration as a dietary supplement. The FDA considers NAC a drug. The difference has stirred up years of controversy because the FDA bans the sale of NAC as a remedy for everything from hangovers to Alzheimer's disease. It also can be confusing, because L-cysteine by itself is an approved supplement.

Possible Side Effects

Little is known about the safety of L-cysteine products used over the long term. There is some concern that taking L-cysteine at the same time as other drugs that suppress the immune system, like prednisone, may increase the power of those drugs and cause negative effects.

The safety of L-cysteine in pregnant women, nursing mothers, and children is not known.


It's too soon to say that L-cysteine products can treat any health condition. It's important to note that self-treating a chronic issue—especially a serious illness such as COPD or heart disease—while avoiding or delaying a doctor visit can have serious health impacts. If you're thinking of using L-cysteine, talk to your doctor about the risks and benefits first.

Dosage and Preparation

L-cysteine is available as a dietary supplement in capsule and powder form. It is often found in protein powders, including whey- and plant-based proteins.

There is no standard dosage. Follow the guidelines on the product label.

What to Look For

Widely available for purchase online, L-cysteine products are sold in many natural-foods stores, drugstores, and stores specializing in dietary supplements.

To ensure safety and quality, look for an independent third-party seal on the label, such as NSF International or ConsumerLab. The label should not make any health promises that it can treat or cure a disease, according to FDA guidelines.

Many L-cysteine products are made from chicken feathers. This may be of concern to some, including those who are vegetarian, vegan, or follow a kosher diet. Some products contain L-cysteine derived from whey protein. Whey is dairy, so kosher rules about dairy apply to them.


Our bodies make L-cysteine, and there are healthy amounts found in meat and other high-protein foods. Adding a supplement to these natural sources may help people with diabetes and other health issues, but the science isn't settled.

Research continues into how L-cysteine and related amino acids may offer health benefits. Be sure to talk with your doctor about side effects, drug interactions, and other concerns before adding L-cysteine to your diet.

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6 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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