First isolated as a chemical compound in 1828, nicotine is a clear, naturally occurring liquid that turns brown when burned and smells like tobacco when exposed to air. It is found in several species of plants, including tobacco and, perhaps surprisingly, in tomatoes, potatoes, and eggplant (though in extremely low quantities that are pharmacologically insignificant for humans). In tobacco, the highest concentration of nicotine appears in the plant's topmost leaves. A poisonous alkaloid, nicotine at high dosages has been used in everything from insecticides to darts designed to bring down elephants.
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