An alkaloid which is the active principle of tobacco (C10H14N2). It occurs in tobacco plants (Nicotiana tabacum and Nicotiana rusticum) to the extent of 2 to 8%, in combination with malic acid or citric acid. It is a colorless, transparent, oily liquid, having an acrid odor, and an acrid burning taste. It is intensely poisonous. The apparently addictive effects of tobacco smoking have been ascribed largely to the effect of nicotine, and the controlled administration of nicotine on various forms has been used as a technique for assisting efforts to stop the smoking habit.