[7] Nicotine is a naturally produced alkaloid in the nightshade family of plants (most predominantly in tobacco and Duboisia hopwoodii)[9] and is widely used recreationally as a stimulant and anxiolytic.

[16] Nicotine is also present at ppb concentrations in edible plants in the family Solanaceae, including potatoes, tomatoes, and eggplants,[17] though sources disagree on whether this has any biological significance to human consumers.

[23][24][25][26] Animal research suggests that monoamine oxidase inhibitors present in tobacco smoke may enhance nicotine's addictive properties.

[39][40] The Surgeon General of the United States indicates that evidence is inadequate to infer the presence or absence of a causal relationship between exposure to nicotine and risk for cancer.

[44] High doses are known to cause nicotine poisoning, organ failure, and death through paralysis of respiratory muscles,[41][45] though serious or fatal overdoses are rare.

Controlled levels of nicotine are given to patients through gums, dermal patches, lozenges, inhalers, or nasal sprays to wean them off their dependence.

A 2018 Cochrane Collaboration review found high-quality evidence that all current forms of nicotine replacement therapy (gum, patch, lozenges, inhaler, and nasal spray) increase the chances of successfully quitting smoking by 50–60%, regardless of setting.

[58] A 2010 meta-analysis of 41 double-blind, placebo-controlled studies concluded that nicotine or smoking had significant positive effects on aspects of fine motor abilities, alerting and orienting attention, and episodic and working memory.

[38] However, at sufficiently high doses, nicotine may result in nausea, vomiting, diarrhea, salivation, bradycardia, and possibly seizures, hypoventilation, and death.

Additionally, nicotine causes systemic vasoconstriction, including constriction of coronary arteries, which can reduce blood flow to the heart.

"[93] A 2018 Cochrane review found that, in rare cases, nicotine replacement therapy can cause non-ischemic chest pain (i.e., chest pain that is unrelated to a heart attack) and heart palpitations, but does not increase the incidence of serious cardiac adverse events (i.e., myocardial infarction, stroke, and cardiac death) relative to controls.

[25] Animal research suggests that monoamine oxidase inhibitors, acetaldehyde[97][98] and other constituents in tobacco smoke may enhance its addictiveness.

[105][106] Contrary to popular belief, nicotine itself does not cause cancer in humans,[40][108] although it is unclear whether it functions as a tumor promoter as of 2012[update].

[109] A 2018 report by the US National Academies of Sciences, Engineering, and Medicine concludes, "[w]hile it is biologically plausible that nicotine can act as a tumor promoter, the existing body of evidence indicates this is unlikely to translate into increased risk of human cancer.

[122] A McMaster University research group observed in 2010 that rats exposed to nicotine in the womb (via parenteral infusion) later in life had conditions including type 2 diabetes, obesity, hypertension, neurobehavioral defects, respiratory dysfunction, and infertility.

[27][29] The initial symptoms of a nicotine overdose typically include nausea, vomiting, diarrhea, hypersalivation, abdominal pain, tachycardia (rapid heart rate), hypertension (high blood pressure), tachypnea (rapid breathing), headache, dizziness, pallor (pale skin), auditory or visual disturbances, and perspiration, followed shortly after by marked bradycardia (slow heart rate), bradypnea (slow breathing), and hypotension (low blood pressure).

[43] At sufficiently high doses, somnolence (sleepiness or drowsiness), confusion, syncope (loss of consciousness from fainting), shortness of breath, marked weakness, seizures, and coma may occur.

[8][43] Lethal nicotine poisoning rapidly produces seizures, and death – which may occur within minutes – is believed to be due to respiratory paralysis.

[134] Nicotine is unusual in comparison to most drugs, as its profile changes from stimulant to sedative with increasing dosages, a phenomenon known as "Nesbitt's paradox" after the doctor who first described it in 1969.

[142][143] Nicotine also activates the sympathetic nervous system,[144] acting via splanchnic nerves to the adrenal medulla, stimulating the release of epinephrine.

[141][157] POMC neurons are a precursor of the melanocortin system, a critical regulator of body weight and peripheral tissue such as skin and hair.

[163]  Conversely, the most common chemistry synthetic methods for generating nicotine yields a product that is approximately equal proportions of the S- and R-enantiomers.

Urinary or salivary cotinine concentrations are frequently measured for the purposes of pre-employment and health insurance medical screening programs.

Nicotine is a secondary metabolite produced in a variety of plants in the family Solanaceae, most notably in tobacco Nicotiana tabacum, where it can be found at high concentrations of 0.5 to 7.5%.

[180] Nicotine is also found at low concentrations in the nectar of tobacco plants, where it may promote outcrossing by affecting the behavior of hummingbird pollinators.

[81] Nicotine was originally isolated from the tobacco plant in 1828 by chemists Wilhelm Heinrich Posselt and Karl Ludwig Reimann from Germany, who believed it was a poison.

[184][185] Its chemical empirical formula was described by Melsens in 1843,[186] its structure was discovered by Adolf Pinner and Richard Wolffenstein in 1893,[187][188][189][clarification needed] and it was first synthesized by Amé Pictet and A. Rotschy in 1904.

[194] However, recently synthetic nicotine started to be found in different brands of e-cigarettes and oral pouches and marketed as "tobacco-free.

The Sunak government proposed banning disposable vapes to limit their appeal and affordability for children and to reduce the amount of waste generated.

A recent systematic review concluded that nicotine was unlikely to be cytotoxic to oral cells in vitro in most physiological conditions but further research is needed.