NAC (N-acetyl-L-cysteine) is an antioxidant that helps increase glutathione synthesis and potentially has benefit during oxidative stress. NAC is the acetyl derivative of L-cysteine. While L-cysteine plays important metabolic roles as a key antioxidant, a glutathione precursor and a natural source of sulfur for metabolism it is unstable and can become degraded during absorption.
NAC on the other hand is more stable than L-cysteine. Taken orally NAC converts into L-cysteine after being absorbed and raises blood and tissue cysteine levels. As a dietary supplement NAC is a highly attractive alternative to L-cysteine.