Acetylcysteine, also known as N-acetylcysteine or N-acetyl-L-cysteine (NAC), is a medication used to treat paracetamol (acetaminophen) overdose and to loosen thick mucus such as in cystic fibrosis or chronic obstructive pulmonary disease. It is available by intravenous, by mouth, or inhaled as a mist. Acetylcysteine was initially patented in 1960 and licensed for use in 1968. It is on the World Health Organization's List of Essential Medicines, most important medication needed in a basic health system. It is available as a generic medication and is not very expensive.