Nicotine is a naturally occurring alkaloid found in many plants. The principal sources of nicotine exposure is through the use of tobacco, nicotine containing gum and nicotine replacement therapies. Nicotine is an amine composed of pyridine and pyrrolidine rings. It has been shown that nicotine crosses biological membranes and the blood brain barrier easily. The absorbed nicotine is extensively metabolized in the liver to form a wide variety of metabolites including nicotine N'-oxide and cotinine N'-oxide. These are the products of mixed function oxidase system. Nicotine is also converted to some biologically important compounds during harvesting. Among these are the nitrosamines specific to tobacco. Nicotine has been shown to affect a wide variety of biological functions ranging from gene expression, regulation of hormone secretion and enzyme activities. The objective of this study was to overview the biological effects and metabolism of nicotine.