Nicotine is an antiherbivory alkaloid and potent parasympathomimetic stimulant found in plants, mostly those in the nightshade family. Nicotine acts as a 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 highly addictive. It is one of the most commonly consumed 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. The evidence suggests that exposure to nicotine between the ages of 10 and 25 years causes lasting harm to the brain and cognitive ability. Nicotine use during pregnancy increases the child's risk of type 2 diabetes, obesity, hypertension, neurobehavioral defects, respiratory problems, and infertility.Nicotine use as a tool for quitting smoking has a good safety history. Nicotine use in the form of nicotine replacement products poses less of a cancer risk than smoking. There is inadequate data to establish if nicotine is itself a carcinogen, but there is evidence of possible risks. The health effects of long-term nicotine replacement use were unknown as of 2014. Overdose on nicotine-containing products results in nicotine poisoning. 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.
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