N-acetylcysteine (NAC) is a supplement form of cysteine, a semi-essential amino acid. It’s recognized for its role in replenishing glutathione, one of the body’s most important antioxidants, and for helping break down mucus in the lungs. NAC is often used in medical settings to treat acetaminophen (Tylenol) poisoning and to loosen thick mucus in individuals with cystic fibrosis or chronic obstructive pulmonary disease. Additionally, it may have benefits for brain health.