Nicotine

Nicotine is the drug in tobacco leaves. Whether someone smokes, chews, or sniffs tobacco, he or she is delivering nicotine to the brain. 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. A drop of pure nicotine would kill a person-in fact, nicotine can be used as a pesticide on crops.

(Excerpts taken teens.drugabuse.gov)