Nicotine is a tertiary amine which is composed of a pyridine and a pyrrolidine ring. Nicotine undergoes a large first pass effect during which the liver metabolizes 80% to 90%; to a smaller extent, the lung ... is able to metabolize nicotine. The major metabolite of nicotine is cotinine; nicotine-1'-N- oxide is a minor metabolite. Cotinine is also extensively metabolized and trans-3'-hydroxycotinine is its a major metabolite. The most abundant metabolite in the mice is trans- 3'-hydroxy-cotinine, accounting for almost 40%, whereas cotinine itself accounts for only about 15% of the dose of nicotine. Cotinine levels in various biological fluids are widely used to estimate intake of nicotine in tobacco users.