N-acetyl cysteine (NAC) is a derivative of the amino acid L-cysteine, and acts as a potent antioxidant. Traditionally, the medication has been used as a decongestant due to its ability to break mucous disulfide bonds and decrease mucous viscosity, as well as an antidote to paracetamol and heavy metal poisoning. NAC is generally well tolerated. Most common side effects involve diarrhoea, nausea and vomiting and unpleasant taste. The medication is taken by patients as eye drops or capsules.