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Quest Vitamins LTD,
8 Venture Way,
Aston Science Park,
B7 4AP.

Tel: 0121 359 0056
Fax: 0121 359 0313
Registered in England No. 2530437



L-cysteine is a non-essential amino acid and can be made in the body from L-methionine and L-serine. The main feature of L-cysteine is that it is a Sulphur-containing amino acid. L-cysteine contributes to protein structure through the cross-linking that occurs between the sulphur components.


In combination with other nutrients, L-cysteine is a potent antioxidant, protecting cells against the effects of radiation, Smoking and other toxins such as alcohol and heavy metals.
L-cysteine is a major part of the peptide gluthathione, which in turn is a component of the antioxidant enzyme, gluthathione peroxidase.

L-cysteine is also necessary for the production of coenzyme A - a vital enzyme in energy production from food. It is additionally a component in the blood sugar-regulating compound, glucose tolerance factor.

Furthermore, L-cysteine is an important structural component of hair. The growth and health of hair depends on an adequate L-cysteine intake.


L-cysteine may be used for:

  • Bronchial conditions (L-cysteine helps to liquefy and clear mucus from the lungs and bronchial tubes (1).
  • Radiation protection - especially in the context of medical treatments (1).
  • Protection against side effects of chemotherapy treatment (1).
  • High cobalt and Molybdenum levels (L-cysteine and/or L-methionine) have been shown to alleviate the symptoms of excessive levels of these metals and may also be useful in other cases of heavy metal toxicity (1).
  • Hair loss and poor hair growth (1).


Although 3-4g daily can be taken without side effects, it is not normally necessary to take more than 1g daily. Over 7g of L-cysteine can lead to the formation of L-cystine Kidney stones. High doses of L-cysteine should ideally be taken with a Vitamin C supplement as this may help to prevent the oxidation of L-cysteine to L-cystine.


Diabetes and Cystinuria:
L-cysteine should be avoided by diabetics. People with the hereditary disorder cystinuria (in which large amounts of L-cystine and other Amino Acids are excreted in the urine), should also avoid L-cysteine supplements.

N.B. L-cysteine should not be confused with L-cystine, which has completely different properties and can be harmful if allowed to build up in the body. L-cystine is the oxidised form of L-cysteine.


1. "The Healing Nutrients Within", E R Braverman & C C Pfeiffer, Keats, 1987.

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