N-acetylcysteine is used as a drug to treat overdose of Paracetamol (Acetaminophen). When Paracetamol is metabolised by the body it is broken down via different pathways into different metabolites. Roughly 60% is metabolised via glucuronidation, 35% via sulfation and 5% via cytochrome P450 pathways. The latter converts Paracetamol into a toxic intermediate, NAPQI (N-acetyl-p-benzoquinone imine). In small amounts, such as with an ordinary dose of paracetamol, the NAPQI is quickly detoxified by conjugation with glutathione into a non-toxic metabolite which can be excreted out through the kidneys.
However, when an overdose of Paracetamol is taken, the NAPQI produced can quickly use up the available of glutathione in the liver, meaning there is a lot of unmetabolised NAPQI left over. This is extremely toxic to liver tissue, as well as being a strong biochemical oxidizer and can cause liver damage and eventually death if left untreated.
N-acetylcysteine is given as a treatment for Parcetamol overdose as it is a precursor to glutathione and hence helps to restore quantities of the substance in the liver. This means remaining NAPQI can be broken down to avoid damage to the liver.