Cysteine: Importance, Toxicity, Uses, Dietary Sources, Precautions and Interactions

It is an amino acid that contains sulfur and an important structural and functional component of proteins and enzymes.

When taken as a supplement, it usually comes in the form of N-acetyl-L-cysteine ​​(NAC).

The N-acetyl-cysteine ​​form is used in medicine as a way to increase the effectiveness of corticosteroids.

It can also be used to decrease the unpleasant symptoms of certain chemotherapy drugs, such as a treatment for paracetamol poisoning and as a way to prevent physiological tolerance to medication for chest pain with nitroglycerin.

Consuming it in both food and dietary supplements has alternative medicine benefits.

It is believed to help protect against cellular damage by free radical compounds, removes potentially harmful metal and chemical ions from body tissues and helps treat or prevent respiratory problems such as asthma and excessive mucus buildup.

The body converts it into cysteine ​​and then into glutathione, a powerful antioxidant. Antioxidants fight free radicals, which are harmful compounds in the body that damage cell membranes and DNA.

Researchers believe that free radicals play a role in aging, as well as in the development of a range of health problems, including heart disease and cancer.

N-acetyl cysteine ​​can help prevent side effects caused by reactions to drugs and toxic chemicals, and helps break down mucus in the body. It seems to have benefits in the treatment of some respiratory conditions, such as bronchitis and COPD.

All proteins are composed of the same basic compounds, known as amino acids. Cysteine, an amino acid that can be synthesized by the body or consumed in certain foods.

Cysteine ​​is found naturally in a number of foods, including egg yolks, red peppers, garlic, onions, yogurt, grains such as wheat germ and oats, poultry and green leafy vegetables such as Brussels sprouts and broccoli.

As a dietary supplement, cysteine ​​is available as L-cysteine ​​hydrochloride and N-acetyl-cysteine, the latter of which is believed to be more soluble and can be metabolized by the body.

Cysteine ​​has several physiological functions, although many people believe that taking additional cysteine ​​as a dietary supplement can also benefit their health. However, there are some cases in which taking cysteine ​​can be dangerous.

Cysteine ​​supplementation is not known to prevent or treat any medical condition and should be used only under the supervision of your doctor.


According to public health recommendations published by the National Academy of Sciences, anyone over 1 year of age should consume 25 mg of cysteine ​​for every gram of protein consumed.

According to eVitamins, however, not enough research has been done with supplemental cysteine ​​to determine the appropriate recommended levels. Two hundred mg of cysteine ​​taken two or three times a day is considered a safe amount.

The “SH” in the abbreviation for glutathione “GSH” represents the sulfur-sulfur group that is critically active. Cysteine ​​is an amino acid that contains sulfur that contributes to the sulfhydryl group in the glutathione molecule.

This makes cysteine ​​the most crucial of the three basic components of glutathione.

This means that the level of cysteine ​​in your system is the limiting factor in how quickly you can produce glutathione and how much it can produce.

When cells have cysteine, they can produce glutathione. Low levels of this important amino acid can reduce its ability to prevent free radical damage and can cause impaired immune system function.

Unfortunately, cysteine ​​is deficient in many diets. In addition to the low amounts of cysteine ​​that are present in our diets, only cysteine ​​in a specific form can enter the cell.

In order to determine the best source of cysteine ​​to build intracellular glutathione, let’s take a closer look at the sources of this crucial amino acid and how the body uses them.

Cysteine ​​Produced by the Body

Your body can produce, and does, some cysteine ​​by itself from another amino acid, methionine , which is also an amino acid that contains sulfur. Methionine is an essential amino acid, which means that it is not produced by the body but comes only from the diet.

The food sources of methionine are all meat and poultry, fish, eggs, dairy products, quinoa, buckwheat, sesame seeds, Brazil nuts and, to a lesser extent, dry spirulina.

While methionine can also be found in other foods, such as beans and legumes, the amount of methionine is too low to be beneficial for the significant production of cysteine ​​and, ultimately, the production of glutathione for your immune health .

The process of transforming methionine into cysteine ​​is a multi-step process, very complex and requires the presence of certain enzymes and B vitamins.

Any small deficiency in this long and complicated chain of events, from consumption to enough glutathione to stimulate the immune system, and stays with a compromised immune system that lacks enough glutathione for the multiple cloning of “T” cells to occur.

The process of transforming methionine into cysteine ​​can be interrupted by several things. Liver diseases and altered metabolism are the most damaging to this process, and this process is completely non-existent in newborn babies.

Fortunately for babies, breast milk is loaded with bound cysteine ​​that contains sulfur.

Part of the cysteine ​​that is produced from methionine can be used to make glutathione inside the cells. Unfortunately, methionine also converts to homocysteine ​​in the body.

Homocysteine ​​has been identified as a high risk factor for the hardening of the arteries (atherosclerosis). For this reason, methionine can not be considered the main source of cysteine ​​for the construction of glutathione, and supplemental methionine (L-methionine) should be avoided.

Cysteine ​​is an amino acid that enters the body in two ways: first, through foods that contain cysteine ​​and second, through a metabolic pathway that converts the amino acid methionine into S-adenosyl methionine, into homocysteine ​​which then reacts with serine and cysteine

The body’s ability to produce cysteine ​​can be affected if the diet does not contain sufficient amounts of folic acid, vitamin B6, methionine and vitamin B12.

In the body, cysteine ​​is an essential part of glutathione, an antioxidant compound, and is also used to produce the amino acid taurine, as well as coenzyme A, biotin and heparin.

Cysteine ​​is a component of beta-keratin and is thought to preserve the elasticity of the skin and protect the lining of the digestive system.

Cysteine ​​is an amino acid, which functions as a protein building block. When used as a supplement, cysteine ​​is generally in the form of N-acetyl-L-cysteine ​​or N-acetyl cysteine.

Your body converts N-acetyl cysteine ​​and then into an antioxidant called glutathione. Antioxidants fight against harmful substances called free radicals, which destroy cell membranes and DNA.

Despite its benefits, it should be taken only after consulting your doctor, as it has the potential to cause side effects.


Extremely high doses (more than 7 grams) can be toxic to the cells in your body and cause death. You should also avoid other forms of cysteine, such as D-cystine, 5-methylcysteine ​​and D-cysteine, since they are also toxic.


N-acetyl cysteine ​​(NAC) can be used to prevent or treat the following conditions:

Paracetamol poisoning:

Doctors often give N-acetyl cysteine ​​intravenously (IV) to people who have taken an overdose of acetaminophen (Tylenol) to help prevent or reduce damage to the liver and kidneys.

Paracetamol poisoning can also occur at lower doses if someone drinks alcohol or takes medications that can damage the liver regularly. Paracetamol poisoning is a medical emergency and can occur due to an accidental overdose. If you think someone has taken an overdose of paracetamol, take them to the hospital.


In clinical studies of people with continuous chest pain, taking N-acetyl cysteine ​​along with nitroglycerin , a drug that opens blood vessels and improves blood flow, has been more effective than taking one to reduce chest pain, the attack heart disease and the risk of death.

However, the combination can also cause a severe headache. You should not try to treat chest pain on your own. Always see a doctor

Chronic bronchitis and chronic obstructive pulmonary disease (COPD):

A review of clinical studies found that N-acetyl cysteine ​​may help relieve the symptoms of chronic bronchitis, resulting in fewer outbreaks. But not all studies agree. A large well-designed study found no reduction in outbreaks.

In another study of people with moderate to severe COPD, taking N-acetyl cysteine ​​reduced the number of outbreaks by about 40% when used with other therapies.


In a 6-month study, people who took 600 mg of N-acetyl cysteine ​​twice a day had fewer flu symptoms than those who took placebo.

Acute respiratory distress syndrome (ARDS):

Acute respiratory distress syndrome (ARDS) occurs after a lung injury and is potentially fatal.

Some studies suggest that intravenous N-acetyl cysteine ​​can increase glutathione levels and help prevent and / or treat lung damage caused by ARDS. However, the results of other studies have been conflicting.

In one study, administering N-acetyl cysteine ​​or procysteine, a synthetic amino acid, to people with ARDS helped reduce the severity of their condition. But it did not reduce the amount of total deaths compared to placebo. ARDS is a medical emergency. You should not try to treat it at home.


Researchers have analyzed whether cysteine ​​can help increase glutathione levels in people with HIV or AIDS.

In a well-designed study, people with HIV who took daily supplements included the amino acid glutamine (40 g per day), vitamin C (800 mg), vitamin E (500 IU), beta carotene (27,000 IU), selenium (280 mcg) ) and N-acetylcysteine ​​(2400 mg) gained more weight after 12 weeks than those who took placebo.

In a small-scale clinical trial where patients with HIV took N-acetyl cysteine, the supplement increased glutathione levels compared to placebo. Other studies have had negative results. More research is needed to see if N-acetyl cysteine ​​has any benefit for people with HIV .

Other uses:

N-acetyl cysteine ​​has also been proposed for the following conditions, although there is not much evidence:

  • Reduce the symptoms associated with Sjögren’s syndrome , an autoimmune disorder that causes dry mouth and dry eyes.
  • Reduce the symptoms of asthma , cystic fibrosis and emphysema .
  • Preventing colon cancer .
  • It helps increase fertility when taken together with fertility drugs in people with polycystic ovarian disease.
  • It helps treat schizophrenia.
  • Reduce the risk of lung cancer among smokers.
  • It helps control blood sugar levels among people with type 2 diabetes.

More studies are needed.

Dietary Sources

Your body produces cysteine ​​from methionine, an essential amino acid. Cysteine ​​is also found in most protein-rich foods, which include:

  • Ricotta.
  • Cottage cheese.
  • Yoghurt.
  • Pork.
  • Sausage meat.
  • Chicken.
  • Turkey.
  • Duck.
  • Stiff.
  • Wheat germ
  • Granola.
  • Avena.

It is present in all foods rich in proteins: all meat and poultry, dairy and eggs, quinoa, buckwheat.

Small amounts of cysteine ​​can be found in other plant sources: broccoli, Brussels sprouts, red and yellow peppers, onions, garlic.

In food, cysteine ​​binds to protein molecules through amide bonds (peptide). The high temperatures during cooking break these bonds and destroy the bioactivity of cysteine.

If the vegetable sources of cysteine ​​are consumed raw, the rough stomach acids break down these bonds.

Free cysteine ​​is trapped by the stomach and intestinal bacteria (they also need it!), Or if the free cysteine ​​survives the trip to the bloodstream, it can not enter the cells.

For cysteine ​​to avoid stomach acids, one will have to eat about 50 pounds. of raw vegetables a day, or – 4-5 lbs. of raw meat, which is out of the question.

And there is a food source out there with very interesting qualities where the cysteine ​​molecule remains intact during digestion.

Raw cow’s milk (or rather, milk whey) contains three highly bioactive proteins: lactoferrin, serum albumin and lactalbumin alfa. These proteins contain exceptional amounts of cysteine.

And most importantly, in the way it can enter cells: each cysteine ​​molecule binds to another molecule of cysteine ​​via a disulfide bridge or bridge:

This paired unit is now called cystine (note the difference in spelling). Cystine can easily enter the cell where it breaks down into two molecules of cysteine ​​and participates in the formation of glutathione.

These disulfide bonds are very fragile and are easily denatured by heat and mechanical stress. Pasteurizing the milk several times at high temperatures before it reaches your table, as well as mechanical stress during centrifugation, destroy these bonds.

It makes the milk cysteine ​​in supermarkets (and all conventional dairy products) absolutely useless for the construction of glutathione. Humans made fun of themselves by ruining the only viable source of cysteine.

For generations, unprocessed milk, cheeses and yogurts used to be the sources of our ancestors of this critical glutathione precursor.

Available forms

  • N-acetyl aerosol (prescription).
  • N-acetyl liquid solution (prescription).
  • N-acetyl topical solution.
  • L-cysteine ​​powder.
  • Cysteine ​​/ N-acetyl tablets or capsules.

How to take it


DO NOT administer N-acetyl cysteine ​​to a child, except under the supervision of a physician.


The recommended adult doses of N-acetyl cysteine ​​vary depending on the health condition being treated. Talk to your doctor to find the safest and most effective dose of N-acetyl cysteine ​​for your condition.

Adding a multivitamin will ensure you are getting the B vitamins you need when you take N-acetyl cysteine.


Because of the possibility of side effects and drug interactions, take dietary supplements only under the supervision of a knowledgeable healthcare provider.

Toxic forms of cysteine ​​that should be avoided include:

  • D-cysteine.
  • D-cystine
  • 5-methyl cysteine.

N-acetyl cysteine ​​can raise levels of homocysteine, an amino acid that is associated with heart disease. Make sure your doctor verifies your homocysteine ​​level if you are taking N-acetyl cysteine.

Very high doses (more than 7 grams) of cysteine ​​can be toxic to human cells and can even cause death.

Taking N-acetyl cysteine ​​by mouth can cause nausea, vomiting and diarrhea.

Intravenous administration of N-acetyl cysteine ​​to treat paracetamol poisoning can cause severe allergic reactions, including:

  • Angioedema.
  • Swelling of the soft tissue just under the skin, which includes the face, lips and around the eyes.
  • Anaphylaxis , a life-threatening allergic reaction

People with cystinuria, a kidney condition in which too much cysteine ​​is lost in the urine, should not take cysteine ​​supplements.

When inhaled into the lungs, N-acetyl cysteine ​​can cause tightness in the chest, numbness of the mouth, runny nose and drowsiness. It can make asthma symptoms worse. People with asthma who take N-acetyl cysteine ​​should be watched closely by their doctors.

Taking large doses of cysteine, particularly in its N-actetylcysteine ​​form, can cause allergic reactions in some people and digestive problems such as diarrhea, vomiting or nausea in others.

According to the world’s healthiest foods, it is believed that people who can not properly metabolize cysteine ​​have a higher risk of developing certain neurological disorders such as multiple sclerosis or Alzheimer’s disease.

Possible Interactions

If you are currently being treated with any of the following medications, you should not use cysteine ​​supplements without first talking to your doctor.

Medications that suppress the immune system:

Treatment with N-acetyl can strengthen the effects of some drugs that suppress the immune system, such as azathioprine (Imuran), cyclophosphamide (Cytoxan) or prednisone (Deltasone). Do not take N-acetyl cysteine ​​with these medicines without first talking to your doctor.

Nitroglycerin and isosorbide:

N-acetyl cysteine ​​can strengthen the effect of nitroglycerin and isosorbide (Isordil), two drugs commonly used to treat chest pain.

But this combination can also increase the risk of side effects, such as severe headaches, and can lead to abnormally low blood pressure. Do not take N-acetyl cysteine ​​with these medications unless your doctor tells you to.


The use of N-acetyl in the skin reinforces the effect of oxiconazole (Oxistat), an antifungal medication used for athlete’s foot.

Activated carbon:

It can make N-acetyl less effective.