NAC (N-acetyl cysteine) comes from the amino acid L-cysteine. 
N-acetyl cysteine is an antioxidant that is commonly used by healthcare providers to treat acetaminophen (Tylenol) poisoning. It works by binding the poisonous forms of acetaminophen that are formed in the liver. People commonly use N-acetyl cysteine for cough and other lung conditions, especially used for winter illnesses, by boosting the body's defence.