Nicotine is a potent parasympathomimetic stimulant and an alkaloid found in the nightshade family of plants. Nicotine acts as a receptor agonist at most nicotinic acetylcholine receptors (nAChRs),^[4][5] except at two nicotinic receptor subunits (nAChRα9 and nAChRα10) where it acts as an receptor antagonist.^[4] Nicotine is found in the leaves of Nicotiana rustica in amounts of 2–14%, the tobacco plant Nicotiana tabacum, Duboisia hopwoodii and Asclepias syriaca.^[6]

Nicotine constitutes approximately 0.6–3.0% of the dry weight of tobacco.^[7] Less than one millionth of that concentration (2–7 µg/kg) is found in edible Solanaceae such as eggplants and tomatoes.^[8] It functions as an antiherbivore chemical; consequently, nicotine was widely used as an insecticide in the past^[9][10] and neonicotinoids such as imidacloprid are currently widely used.

Nicotine is highly addictive.^[11][12] An average cigarette yields about 2 mg of absorbed nicotine, and in lesser doses of that order, the substance acts as a stimulant in mammals, while high amounts (50–100 mg) can be harmful.^[13][14][15] This stimulant effect is a contributing factor to the addictive properties of tobacco smoking. Nicotine's addictive nature includes psychoactive effects, drug-reinforced behavior, compulsive use, relapse after abstinence, physical dependence and tolerance.^[16]

Beyond addiction, both short and long-term nicotine exposure have not been established as dangerous to adults,^[17] except among certain vulnerable groups.^[18] At high-enough doses, nicotine is associated with poisonings and is potentially lethal.^[15][19] Nicotine as a tool for quitting smoking has a good safety history.^[20] There is inadequate research to show that nicotine itself is associated with cancer in humans.^[19] Nicotine in the form of nicotine replacement products is less of a cancer risk than smoking.^[19] Nicotine is linked to possible birth defects.^[21] During pregnancy, there are risks to the child later in life for type 2 diabetes, obesity, hypertension, neurobehavioral defects, respiratory dysfunction, and infertility.^[20] The use of electronic cigarettes, which are designed to be refilled with nicotine-containing e-liquid, has raised concerns over nicotine overdoses, especially with regard to the possibility of young children ingesting the liquids.^[22]