N-acetyl cysteine (NAC) is a supplement that has many health benefits. It is a precursor of L-cysteine that results in glutathione elevation biosynthesis, which is one of the most potent antioxidants in the body. NAC is used by the body to build antioxidants, which protect and repair cells from damage. It is also used as a medicine to treat acetaminophen (Tylenol) poisoning. NAC can help prevent or reduce liver and kidney damage caused by an acetaminophen overdose. It can also help prevent side effects caused by drug reactions and toxic chemicals, and helps break down mucus in the body. NAC seems to have benefits in treating some respiratory conditions, such as bronchitis and COPD. In clinical studies of people with ongoing chest pain, taking NAC along with nitroglycerin, a drug that opens up blood vessels and improves blood flow, has been more effective than taking either one alone in reducing chest pain, heart attack, and the risk of death. However, the combination can also cause a severe headache, and chest pain should always be treated by a doctor. NAC may also help relieve symptoms of chronic bronchitis, leading to fewer flare-ups. It has been used for treatment of various diseases in a direct action or in combination with some other medications. Although NAC is available as an over-the-counter nutritional supplement, optimal doses of NAC as a supplement have not been set for any condition, and quality and active ingredients in supplements may vary widely from maker to maker. Therefore, it is important to ask a doctor for advice before taking NAC as a supplement.