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, the body’s most important intracellular antioxidant and a vital detoxifier.

N-acetyl-L-cysteine (NAC) is the acetylated form of L-cysteine that has been used to break up pulmonary and bronchial mucous. It is very efficiently absorbed and has been shown to protect cells directly and support the immune system. NAC is an antioxidant and a free radical-scavenging agent that increases intracellular glutathione at the cellular level. NAC can act as a precursor for glutathione synthesis as well as a stimulator of the cytosolic enzymes involved in glutathione regeneration. NAC has been shown to protect against oxidative stress-induced neuronal death in cultured granule neurons.