Why We Take Cysteine Rather Than N-Acetylcysteine

The Durk Pearson & Sandy Shaw®
Life Extension NewsTM
Volume 5 No. 5 • October 2002


Why We Take Cysteine Rather Than N-Acetylcysteine

N-Acetylcysteine is a xenobiotic (not normally found in the body), although it is an acetylated form of the natural amino acid cysteine. It can be sold as a dietary supplement even though it is a xenobiotic, because it was being sold as a dietary supplement before passage of the Dietary Supplement Health and Education Act, thereby "grandfathering" it. N-Acetylcysteine is an FDA-approved prescription drug used in the treatment of Tylenol® (acetaminophen) overdose (liver toxicity). N-Acetylcysteine treatment for this purpose lasts up to about 3 days. The treatment works by increasing liver glutathione levels.

Cysteine acts as the limiting natural precursor to glutathione and increases its levels. (Methionine can also act as a precursor to glutathione, but homocysteine is a byproduct of this pathway.) It is easier to get permission to use N-acetylcysteine in human and even animal studies because of its FDA-approved status. Moreover, there is a belief that cysteine is more toxic than N-acetylcysteine because the latter is more water-soluble, and cysteine can be oxidized to cystine. However, taking at least twice as much vitamin C as cysteine prevents oxidation of cysteine to cystine (which can cause cystine stones in the kidneys and urinary bladder).

The acetyl group in N-acetylcysteine is labile, that is, it can acetylate many things indiscriminately. Aspirin, acetylsalicylic acid, also contains an acetyl group, but its acetylation is more specific. Aspirin acetylates a specific amino acid site in the enzyme that synthesizes thromboxane, thus acting as an anticlotting agent. Since N-acetylcysteine has been tested for and approved for a very short-term use, we do not feel that we know enough about it to consider taking it on a long-term, everyday basis.

N-Acetylcysteine can inhibit blood clotting by increasing prothrombin time.1 Hence, in combination with other anticlotting agents, such as low-dose aspirin, ginkgo, or fish oils, blood-clotting time could be excessively extended. Your doctor can easily check your prothrombin time, or it can be inexpensively and safely tested at a walk-in clinical laboratory testing firm.

  1. Pol and Lebray. N-Acetylcysteine for paracetamol poisoning: effect on prothrombin. Lancet 360:1115 (2002).

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