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L-cysteine is a semi-essential amino acid. The body can synthesize it from methionine and serine, however if these amino acid stores are low then dietary intake is considered essential.

A deficiency in l-cysteine is linked to poor metabolic functioning, such as reduced proteome development, signal transduction, biological defenses, and increased rates aging and disease [1].

L-cysteine supplementation is mentioned to have antioxidative properties, and is linked to liver detoxification, elimination of heavy metals, and improved hair health.

It is important to know that although some clinical trials have tested l-cysteine supplementation, most of them analyze n-acetyl-l-cysteine instead – a precursor of l-cysteine. Due to the similarities, this review will include n-acetyl-l-cysteine research to assess the benefits of l-cysteine.

L-Cysteine Health Benefits

A Powerful Antioxidant

L-cysteine can directly and indirectly act as a scavenger of free radicals.

As it is such a powerful antioxidant it is recommended as a potential treatment option for many disorders that stem from the generation of free oxygen radicals [2].

The ability for l-cysteine to act as an antioxidant mainly comes from its role in replenishing glutathione stores – a powerful antioxidant with a lot of good research behind it [3]. In fact, l-cysteine is a rate-limiting amino acid that determines the availability of glutathione [4].

Glutathione participates in a cycle to isolate and neutralize oxygen and nitrogen radicals, where they will eventually be degraded.

Glutathione is essential for fighting cellular damage and is closely linked with longevity [5]. It may also be important for preventing heart disease, infertility, and psychiatric conditions [6].

L-cysteine intake becomes even more essential when considering that directly consuming glutathione has limited therapeutic use as it is rapidly hydrolyzed in the intestines and is not particularly bioavailable [7].

However, some research suggests that only n-acetyl-l-cysteine can increase concentrations of glutathione, not l-cysteine supplementation itself, as first pass metabolism is thought to limit much of the distribution of cysteine to the body. Future research is necessary to clarify this theory.

May Help With Psychiatric Disorders

Persons with obsessive compulsive disorder have increased lipid peroxidation in the brain, associated with a decreased availability of antioxidant enzymes [8].

Some brain damage is caused by the combination of excess glutamate (neurotransmitter in brain) and glutathione deficiency. As l-cysteine helps to regulate levels of glutamate, as well as increase glutathione production, it is not surprising that it may help with psychiatric disorders [9].

L-cysteine supplementation has shown to help individuals with bipolar disease, depression, obsessive compulsive disorder, and anxiety [10] [11] [12] [13].

In addition, animal studies indicate that l-cysteine may reduce the symptoms of schizophrenia, such as social withdrawal, apathy and reduced attention spans [14].

Interestingly, it may even decrease withdrawal symptoms from extremely addictive substances such as nicotine, marijuana, and even cocaine [15] [16].

May Help Chronic Obstructive Pulmonary Disease

Chronic obstructive pulmonary disease is where a person experiences of severe limitation of airflow that is not fully reversible. It tends to be associated with abnormal inflammatory and oxidative responses to stressors, particularly in lung tissue.

Due to the pathology being from oxidative stress, n-acetylcysteine has been investigated for its usage.

In people with chronic obstructive pulmonary disorder, 1200mg of n-acetylcysteine per day was associated with ~20-35% less hydrogen peroxide in their exhaled air, suggesting less oxidative stress in lung tissue [17].

Researchers also suggest that l-cysteine may be able to reduce the amount of mucus in bronchial tubes and increase glutathione levels to effectively reduce wheezing, coughing and respiratory attacks [18].

However, when n-acetylcysteine supplementation has been assessed in the long-term (3 years) it has failed to reduce the deterioration rate of lung function compared to placebo [19].

More long-term human trials are needed to fully understand the implications of cysteine-based supplementation.

How To Take L-Cysteine?

There are currently no specific dietary recommendations for l-cysteine, or well-established supplemental dosages, due to disputes about the bioavailability and absorption rates of the amino acid from various sources.

Despite this, typical supplement recommendations are between 600-1,800mg of l-cysteine or n-acetylcysteine per day, taken orally.

L-Cysteine Safety And Side Effects

L-cysteine is generally regarded as a safe supplement to consume.

If high doses are taken for prolonged periods this may lead to:

  • Nausea
  • Vomiting
  • Diarrhea
  • Constipation

It is not advised for individuals with bleeding disorders, or those taking blood thinning medications, to consume an l-cysteine based supplement as it could potentially slow blood clotting and cause health complications.


L-cysteine is a semi-essential amino acid that acts as an antioxidant metabolic functioning and has roles within biological defenses and the prevention of certain diseases.

Studies on l-cysteine supplementation (or at least n-acetylcysteine) show it may prevent psychiatric disorders and chronic obstructive pulmonary disease through a variety of different mechanisms.

It is recommended to take between 600-1800mg per day to see potential benefits.

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Fact Checked

This article has been reviewed and fact-checked by a certified nutritionist, and only uses information from credible academic sources.