Nicotine is the drug in tobacco leaves. Nicotine constitutes approximately 0.6–3.0% of dry weight of tobacco, with biosynthesis taking place in the roots, and accumulating in the leaves. It functions as an ant herbivore chemical with particular specificity to insects; therefore nicotine was widely used as an insecticide in the past, and currently nicotine analogs such as imidacloprid continue to be widely used. According to the American Heart Association, "Nicotine addiction has historically been one of the hardest addictions to break."

Common Names: smokes, cigs, or butts. Smokeless tobacco is often called chew, dip, spit tobacco, or snuff.

Appearance: Tobacco can be smoked in cigarettes, cigars, or pipes. It can be chewed or, if powdered, sniffed. Whether someone smokes, chews, or sniffs tobacco, he or she is delivering nicotine to the brain.

Effects: Each cigarette contains about 10 milligrams of nicotine. Nicotine is what keeps people smoking despite its harmful effects. Because the smoker inhales only some of the smoke from a cigarette and not all of each puff is absorbed in the lungs, a smoker gets about 1 to 2 milligrams of the drug from each cigarette. This release of neurotransmitters and hormones is responsible for most of nicotine effects. Modern research shows that nicotine acts on the brain to produce a number of effects. Specifically, its addictive nature has been found to show that nicotine activates reward pathways—the circuitry within the brain that regulates feelings of pleasure and euphoria.