NAC (N-acetyl cysteine) is a supplement that may be used to help people replenish glutathione – the most powerful antioxidant in the body. Levels of glutathione rapidly deplete during oxidative stress or after acetaminophen overdosage. NAC may also be used to help improve fertility, brain health, heart health, and long-term respiratory conditions (such as Asthma or COPD) and reduce the risk of developing certain cancers. In medicine, NAC has been used as an antidote to acetaminophen overdose since the 1960s and to treat those with acute liver failure.