L-cysteine is a conditionally essential amino acid, one of only three sulfur-containing amino acids, the others being taurine (which can be produced from L-cysteine) and L-methionine from which L-cysteine can be produced in the body by a multi-step process. Cysteine plays a role in the sulfation cycle, acting as a sulfur donor in phase II detoxification and as a methyl donor in the conversion of homocysteine to methionine. Cysteine also helps synthesize glutathione, one of the body's most important natural detoxifiers. N-acetyl-cysteine is the acetylated form of L-cysteine, which is more efficiently absorbed and used.

Glutathione (gamma-L-glutamyl-L-cysteinyl-glycine) is a peptide (short protein)-like molecule synthesized in the body from the three amino acids L-glutamic acid, L-cysteine, and glycine. Glutathione is one of the body's most important and powerful antioxidants, helping to detoxify xenobiotics. A major function of vitamin C is to keep glutathione, L-cysteine, and N-acetyl-cysteine in reduced form so that they can continue to provide free radical quenching effects.

Even though many published studies show that garlic, selenium, alpha-lipoic acid, L-cysteine, and N-acetyl-cysteine can boost cellular glutathione levels, people wishing to maintain optimal health may benefit from taking high doses of glutathione.*

When taking L-cysteine or N-acetyl-cysteine, it is recommended that three times as much vitamin C be taken at the same time to prevent these amino acids from being oxidized in the body.*