Nicotine is an alkaloid found primarily in plants of the nightshade family, notably in tobacco and Duboisia hopwoodii.[8] Nicotine is used recreationally for its stimulant and anxiolytic effects. In tobacco leaves, nicotine constitutes about 0.6–3.0% of the dry weight,[9] and smaller, trace quantities occur in other Solanaceae crops such as tomatoes, potatoes, and eggplants. In pure form, nicotine is a colorless to yellowish, oily liquid that readily penetrates biological membranes and acts as a potent neurotoxin in insects, where it serves as a antiherbivore toxin. Historically, it was widely used as an insecticide, and its structure provided the basis for synthetic neonicotinoid pesticides.[10] In humans, nicotine acts primarily as a stimulant by binding to and activating nicotinic acetylcholine receptors (nAChRs) in the central nervous system and peripheral tissues. This results in the release of neurotransmitters such as dopamine, acetylcholine, and norepinephrine, producing effects including increased alertness, reduced anxiety, and mild euphoria.[11] Nicotine is typically consumed through tobacco smoking, vaping, or other nicotine delivery systems. An average cigarette yields about 2 mg of absorbed nicotine, a dose sufficient to produce reinforcement and dependence while remaining far below toxic levels.[12] Nicotine is highly addictive, and nicotine dependence is characterized by tolerance, physical dependence, psychological dependence, and nicotine withdrawal symptoms such as irritability, anxiety, and difficulty concentrating.[13][14] Nicotine replacement therapy (NRT) products, including gums, patches, and lozenges, deliver the compound in slower, lower doses that are less addictive and are used medically to help people quit smoking.[15][16] Synthetic derivatives of nicotine, such as varenicline, act as partial agonists at nicotinic receptors and are also used as smoking cessation aids.[17] Although nicotine itself is not classified as a carcinogen by either the International Agency for Research on Cancer or the Surgeon General of the United States,[18] high doses can cause nicotine poisoning and respiratory paralysis. Chronic exposure may also affect the immune, cardiovascular, and nervous systems.[19][20] Nicotine is also a known teratogen, associated with adverse developmental effects during pregnancy,[21] and may impair adolescent neurodevelopment, though the extent of this effect in humans remains debated.[22]

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