Nicotine is a potent parasympathomimetic stimulant and an alkaloid found in the nightshade family of plants. Nicotine acts as an exogenous receptor agonist at most nicotinic acetylcholine receptors (nAChRs), except at two nicotinic receptor subunits (nAChRα9 and nAChRα10) where it acts as a receptor antagonist. Nicotine is found in the leaves of Nicotiana rustica, in amounts of 2–14%; in the tobacco plant, Nicotiana tabacum; in Duboisia hopwoodii; and in Asclepias syriaca.It constitutes approximately 0.6–3.0% of the dry weight of tobacco. Usually consistent concentrations of nicotine varying from 2–7 µg/kg (20–70 millionths of a percent wet weight) are found in the edible family Solanaceae, such as potatoes, tomatoes, and eggplant. Some research indicates that the contribution of nicotine obtained from food is substantial in comparison to inhalation of second-hand smoke. Others consider nicotine obtained from food to be trivial unless exceedingly high amounts of certain vegetables are eaten. It functions as an antiherbivore chemical; consequently, nicotine was widely used as an insecticide in the past, and neonicotinoids, such as imidacloprid, are widely used.
Nicotine is highly addictive. It is one of the most commonly abused drugs. An average cigarette yields about 2 mg of absorbed nicotine, while high amounts (30–60 mg) can be harmful. Nicotine induces both behavioral stimulation and anxiety in animals. Nicotine addiction involves drug-reinforced behavior, compulsive use, and relapse following abstinence. Nicotine dependence involves tolerance, sensitization, physical dependence, and psychological dependence. Nicotine dependency causes distress. Nicotine withdrawal symptoms include depressed mood, stress, anxiety, irritability, difficulty concentrating, and sleep disturbances. Mild nicotine withdrawal symptoms are measurable in unrestricted smokers, who experience normal moods only as their blood nicotine levels peak, with each cigarette. On quitting, withdrawal symptoms worsen sharply, then gradually improve to a normal state.Nicotine use as a tool for quitting smoking has a good safety history. The general medical position is that nicotine itself poses few health risks, except among certain vulnerable groups, such as youth. The International Agency for Research on Cancer indicates that nicotine does not cause cancer. Nicotine has been shown to produce birth defects in some animal species, but not others; consequently, it is considered to be a possible teratogen in humans. The median lethal dose of nicotine in humans is unknown, but high doses are known to cause nicotine poisoning.