Cysteine: Benefits, Uses, and Side Effects

Cysteine is an amino acid with a crucial role in numerous biological processes. It is necessary for healthy protein synthesis and is a key component of skin, hair, and nails. It also promotes antioxidant activity within the body and could help to prevent chronic disease.

However, despite these essential functions, taking a cysteine supplement may not be a good choice for everyone. Read on to discover the benefits and risks of this amino acid and whether it is suitable for you.

What Is Cysteine?

Cysteine is a non-essential amino acid, meaning our bodies can synthesize it, and we do not need to get it from our diets. Several foods do contain cysteine, including poultry, eggs, yogurt, and garlic. However, the body makes most of its cysteine from another amino acid called methionine.

Cysteine is a sulfur-containing amino acid. Its sulfur atom is contained in something known as a thiol group at the end of its side chain. This makes the molecule highly reactive, and it has many vital functions.


There are two forms of cysteine, L-cysteine and N-acetyl cysteine (NAC). L-cysteine is the primary form of cysteine found within the body. Meanwhile, NAC is an FDA-approved compound licensed to treat acetaminophen toxicity. Doctors also use it to treat respiratory conditions in which mucus is a significant factor.

Let’s look closer at how cysteine works and its potential benefits.

How Does Cysteine Work?

As we mentioned, most of the body’s cysteine is created from another amino acid, methionine. This process is called trans-sulfuration, and it takes place in the liver. Vitamins B6, B9 (folate), and B12 are required to convert methionine to cysteine.

After trans-sulfuration occurs, cysteine goes on to form several other essential compounds, including taurine, glutathione, and coenzyme A.

Taurine has antioxidant and anti-inflammatory properties and is involved in visual and neural development. Read our in-depth guide to taurine to learn more.

Glutathione is the body’s master antioxidant. It plays a significant role in reducing oxidative stress, a process in which harmful molecules called reactive oxygen species damage cells.

Oxidative stress is associated with a vast range of chronic conditions, from high blood pressure to Parkinson’s disease. Glutathione helps to delay the onset of these disorders via its antioxidant effects.

Meanwhile, coenzyme A is necessary for numerous cellular reactions, including energy production, fatty acid and cholesterol synthesis, and more. It also has a role in regulating gene expression, making it one of the most important molecules in the body.

In addition to generating these essential compounds, cysteine is involved in collagen production. Collagen is the protein responsible for keeping skin flexible and strong. The amino acid is also a component of beta-keratin, the main protein in skin, hair, and nails.

Finally, cysteine can improve mucolytic function, facilitating mucus and phlegm expulsion. For this reason, it helps treat certain respiratory disorders, including bronchitis and asthma.

With so many potential benefits, it’s little wonder that cysteine use is on the rise. However, clinical trials exploring the impact of supplementation are inconsistent. We have detailed some of the latest research below.

Cysteine Benefits

According to a 2018 review, the benefits of cysteine include:

  • Antioxidant effects
  • Mucolytic effects
  • Strengthening the hair
  • Improving immune function
  • Protecting and detoxifying the liver
  • Heart disease prevention
  • Diabetes prevention
  • Slowing the aging process
  • Protecting the digestive system

However, the paper’s authors noted that the evidence supporting these benefits is inconclusive. While some studies have yielded positive results, others have found no apparent effect. The researchers also pointed out NAC has been studied far more thoroughly than L-cysteine.

Cysteine Side Effects

Although it occurs naturally in the body, cysteine supplements could cause adverse reactions in some individuals. The most common cysteine side effects include:

  • Dry mouth
  • Nausea
  • Vomiting
  • Diarrhea

Furthermore, inhaled products could cause:

  • Runny nose
  • Drowsiness
  • Clammy skin
  • Chest tightness

The amino acid could also interact with some medicines. Therefore, anyone taking prescribed or over-the-counter medication should consult a physician before use. For example, cysteine appears to affect how the body uses insulin and may be unsafe for people with diabetes.

Furthermore, people with a condition called cystinuria should avoid cysteine supplements. It is a genetic disorder that affects cysteine metabolism, causing it to accumulate in the kidneys and bladder. This can lead to kidney stones.

Cysteine Dosage

The most appropriate cysteine dosage depends upon various factors, including the reason for using it and individual physiology. The form of cysteine is also a factor to consider (i.e., L-cysteine vs. NAC).


The National Library of Medicine recommends a daily dose of 600–1200mg for oral NAC. However, it is more challenging to find dosing guidelines for L-cysteine since it is not approved as a medicine. Most commercial products contain 500mg per capsule, which many consider a suitable amount.

We recommend following the dosing guidelines on your product’s packaging or consulting a healthcare provider for further information.

Cysteine Supplement

Most people can synthesize adequate cysteine by consuming a balanced diet with plenty of protein and B vitamins. Therefore, supplements are rarely necessary.

Indeed, there is some evidence that restricting cysteine and methionine intake could be beneficial, meaning supplements may do more harm than good. We recommend that anyone considering supplementation consults a physician to determine whether it is suitable for them.

Individuals who go down this route will find an assortment of different products for sale. Since NAC is an FDA-approved medicine, it is technically illegal to include it in dietary supplements.

Therefore, it may be easier to find products containing L-cysteine. Consumers should keep in mind that this form of the amino acid has not been studied as thoroughly as NAC.

Some brands market cysteine-only supplements while others combine it with other amino acids.

Some sources suggest that the latter may be preferable as single amino acid supplements have been linked with nitrogen imbalances, impaired metabolism, kidney issues, and growth problems in children.

It is also essential to only purchase supplements from reputable brands. Look for companies that are transparent about their manufacturing processes, label their products clearly, and provide up-to-date lab reports.

You can read about some of our top-rated sellers in our Best Amino Acids Review.

The Bottom Line on Cysteine

Cysteine is a non-essential amino acid with many crucial functions in the body. However, the evidence regarding the benefits of supplementation is inconclusive. Moreover, inappropriate use of cysteine supplements could cause various health issues.

We recommend consulting a knowledgeable physician before beginning any new supplement to ensure it is safe and suitable for you.

To learn more about the benefits of amino acids, check out our related articles below:

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