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Overview: L-cysteine is an amino acid that your body uses for many important functions, including building proteins. Your body makes L-cysteine from another amino acid called methionine. You also get L-cysteine from many common foods. L-cysteine dietary supplements appear to be well tolerated and are sometimes used to improve overall health. However, since your body makes L-cysteine on its own, it is not clear if L-cysteine supplements offer any benefits. Before taking L-cysteine, tell your health care provider about all of your health conditions and any medicines or supplements you may be using.

Uses

What is L-cysteine used for?

L-cysteine supplements may be used to improve overall health, but their benefits are not well defined.

Note: The FDA has not reviewed L-cysteine supplements for safety and effectiveness. Learn more about dietary supplements here.

How does L-cysteine work (mechanism of action)?

L-cysteine is an amino acid, which is a building block of protein and other important substances. Your body makes L-cysteine from another amino acid called methionine. You also get L-cysteine from many foods, including meat, fish, dairy, nuts, seeds, soy, and legumes. Some product labels may list L-cysteine as cysteine, which is the same thing.

Your body usually makes enough L-cysteine to meet your needs. However, infants who are very sick may need more L-cysteine than their bodies can make. As a result, some infant formulas and other foods for infants and young children contain extra L-cysteine. But it is not clear if this extra L-cysteine can improve the health of an infant or young child.

Although L-cysteine supplements are sometimes used by adults for many different health purposes, their benefits for any use are not well defined. There is some research under way to understand whether getting smaller amounts of L-cysteine from foods may affect your health and body weight. More research is needed to understand the effects of large and small amounts of L-cysteine in the body.

How should I store L-cysteine?

You should store L-cysteine according to the product label. Most dietary supplements should be kept in a cool, dry location that is out of the reach of children and pets and not in direct sunlight.

Side Effects

What are the most common side effects of L-cysteine?

L-cysteine that comes from foods is well tolerated. But there is not enough research to know what the common side effects of L-cysteine supplements may be. Tell your health care provider if you have any side effects that bother you.

There may be side effects of L-cysteine that are not listed here. Contact your health care provider if you think you are having a side effect of a supplement. In the U.S., you can report side effects to the FDA at www.safetyreporting.hhs.gov or by calling 888-SAFEFOOD (888-723-3366). In Canada, you can report side effects to Health Canada at www.health.gc.ca/medeffect or by calling 866-234-2345.

What are the serious side effects of L-cysteine?

While less common, the most serious side effects of L-cysteine are described below, along with what to do if they happen.

Severe Allergic Reactions. L-cysteine may cause allergic reactions, which can be serious. Stop taking L-cysteine and get help right away if you have any of the following symptoms of a serious allergic reaction.

  • Breathing problems or wheezing
  • Racing heart
  • Fever or general ill feeling
  • Swollen lymph nodes
  • Swelling of the face, lips, mouth, tongue, or throat
  • Trouble swallowing or throat tightness
  • Itching or skin rash
  • Bumps on the skin called hives that can be red, pink, white, or brown depending on your skin tone
  • Nausea or vomiting
  • Dizziness, feeling lightheaded, or fainting
  • Stomach cramps
  • Joint pain

Warnings & Precautions

Who should not use L-cysteine?

Allergies to Ingredients. People who are allergic to any of the following should not take L-cysteine.

  • Cysteine
  • L-cysteine
  • Any of the ingredients in the specific product

Your pharmacist can tell you all of the ingredients in the specific L-cysteine products they stock.

What should I know about L-cysteine before using it?

L-cysteine dietary supplements cannot be marketed to treat or cure a disease.

Read and follow the directions on the specific product you are taking, or take it as recommended by your health care provider.

Keep L-cysteine out of the reach of children.

L-cysteine is similar to another chemical, N-acetylcysteine (NAC), that is also available as a dietary supplement. Your body can make L-cysteine from NAC. However, NAC also has many other functions in the body, and it is not the same as L-cysteine. It is important to read the product label to understand which ingredient it contains.

The FDA has not tested L-cysteine products to confirm that they contain the ingredients stated on their labels. Some dietary supplements have been tested by third-party organizations to confirm that they contain the ingredients listed on their labels and do not contain any harmful chemicals. Learn more about how to choose a safe dietary supplement product here.

What should I tell my health care provider before using L-cysteine?

Tell your health care provider about all of your health conditions and any prescription or over-the-counter (OTC) medicines, vitamins/minerals, herbal products, and other supplements you are using. This will help them know if L-cysteine is right for you.

In particular, make sure that you discuss any of the following.

Current and Past Health Conditions. Tell your health care provider if you have any of the following.

  • A rare condition called cystinuria
  • Kidney problems
  • Liver problems

Pregnancy. It is not known if or how L-cysteine could affect pregnancy or harm a fetus. Tell your health care provider if you are pregnant or plan to become pregnant.

Breastfeeding. L-cysteine is a natural part of breast milk. Tell your health care provider if you are breastfeeding or plan to breastfeed.

Interactions

Does L-cysteine interact with foods or drinks?

There are no known interactions between L-cysteine and foods or drinks.

It is unknown if drinking alcohol will affect L-cysteine.

Does L-cysteine interact with medicines (drug interactions)?

There are no known interactions between L-cysteine and medicines. Always tell your health care provider about any prescription or over-the-counter (OTC) medicines, vitamins/minerals, herbal products, and other supplements you are using.

The interactions between L-cysteine and medicines are not fully understood. As with most dietary supplements, the research on drug interactions with L-cysteine is incomplete. Always check with your health care provider before taking L-cysteine with any medicines.

Overdose/Missed Dose

What should I do if I accidentally use too much L-cysteine?

If you or someone else has used too much L-cysteine, get medical help right away, call 911, or contact a Poison Control center at 800-222-1222.

Read the product label closely and speak with a health care provider about how to take L-cysteine safely. Do not take double or extra doses, even if you miss a dose.