N-Acetylcysteine (NAC) is a medication and supplement that serves as a precursor to the amino acid cysteine. It is commonly used to help replenish levels of glutathione, a powerful antioxidant in the body. NAC is often prescribed to treat acetaminophen overdose and to help with certain lung conditions by thinning mucus. In addition to its medical uses, NAC is also popular as a dietary supplement for its potential benefits in supporting mental health and reducing oxidative stress. Research is ongoing to explore its effects on various health conditions, including chronic obstructive pulmonary disease (COPD) and psychiatric disorders.