N-acetylcysteine (NAC) is a derivative of the naturally occurring amino-acid cysteine. Many people use it as a nutritional supplement, but NAC also has clear pharmacological properties. By far the most important role of NAC is as an ‘antidote’ in cases of paracetamol overdose. In this role, it can be life-saving.
However, the properties of NAC may extend beyond the emergency room. Recent research from Australia suggests it might be effective in the depressive phase of bipolar disorder. http://www.scielo.br/pdf/rbp/v33n4/v33n4a11.pdf
Larger studies are needed, but if the initial promise holds true, this would represent an important advance. In bipolar disorder, the depressive phase can be extremely difficult to shift, although conventional pharmacology has also made some significant inroads in the past few years. http://journals.psychiatryonline.org/data/Journals/AJP/4013/1351.pdf