N-Acetylcysteine (NAC) is an antioxidant naturally found in plants of the Allium species such as onions. As well as having antioxidant and anti-inflammatory properties, it is also needed to make glutathione which is an important antioxidant in cells. NAC has been used as a drug since the 1960s and is a popular dietary supplement. NAC is considered safe but the results of clinical trials are controversial or incomplete. This aim of this review is to provide a balanced view of further uses of NAC as a dietary supplement. It discusses the antioxidant properties in relation to conditions such as heart disease, diabetes and ageing; its medical uses, in the complementary treatment of cancer, as a sports supplement and as an anti-aging supplement. More information is needed on NAC and further clinical trials looking at its use in neurodegenerative diseases, addiction, and mental health disorders are underway. This may be relevant to the supplement users and may also help people suffering from chronic degenerative conditions.