Cysteine is a nonessential amino acid
(protein building block), meaning that cysteine can be made in the human body. Cysteine is one
of the few amino acids that contains sulfur.
This allows cysteine to bond in a special way and maintain the structure of proteins in the
body. Cysteine is a component of the
antioxidantglutathione. The body also uses
cysteine to produce taurine, another amino
Where is it found?
The body can synthesize cysteine from
methionine and other building blocks. Cysteine, the amino acid from which NAC is derived, is found in most
Who is likely to be deficient?
According to several studies, blood levels of cysteine and glutathione are low in people infected with HIV.1 2 3 Cysteine
has a role in the proper function of the immune
system, so a deficiency of this amino acid
may either contribute to, or result from, immune suppression associated with HIV.
How much is usually taken?
Most people do not need to supplement with cysteine. Almost nothing is known about
appropriate supplemental levels, in part because almost all clinical research has been done
with N-acetyl cysteine and not cysteine
Are there any side effects or interactions?
No consistent adverse effects of NAC have
been reported in humans. One small study found that daily amounts of 1.2 grams or more could
lead to oxidative damage.4 Extremely large amounts of cysteine, the amino acid NAC is derived from, may be toxic to nerve
cells in rats.5
Adequate amounts of methionine are needed
in the diet, as the precursor to cysteine, to prevent cysteine deficiency.
At the time of writing, there were no well-known drug interactions
1. Eck HP, Gander H, Hartmann M, et al. Low concentrations of
acid-soluble thiol (cysteine) in the blood plasma of HIV-1 infected patients. Biol Chem
Hoppe Seyler 1989;370:101–8.
2. Droge W, Eck HP, Mihm S. HIV-induced cysteine deficiency and T-cell
dysfunction—a rationale for treatment with N-acetylcysteine. Immunol Today
3. Droge W. Cysteine and glutathione deficiency in AIDS patients: a
rationale for the treatment with N-acetyl-cysteine. Pharmacology 1993;46:61–5
4. Kleinveld HA, Demacker PNM, Stalenhoef AFH. Failure of
N-acetylcysteine to reduce low-density lipoprotein oxidizability in healthy subjects. Eur
J Clin Pharmacol 1992;639–42.
5. Olney JW, Ho OL. Brain damage in infant mice following oral intake of
glutamate, aspartate or cysteine. Nature 1970;227:609–10 [letter].