Nicotine is a naturally occurring compound that is classified as a liquid alkaloid. An alkaloid is an organic compound made up of carbon, hydrogen, nitrogen, and sometimes oxygen (nicotine lacks oxygen). Another example of an alkaloid is caffeine. Both caffeine and nicotine are similar in that they provide stimulation upon ingestion, a factor that encourages their habitual use.

Nicotine can gain entry to the bloodstream by several routes. Tobacco can also be applied as a wad to the inside of the mouth, enabling nicotine to diffuse across the mucous membranes of the gums to the bloodstream. As a component of a cigarette, nicotine can be inhaled into the lungs. There, it encounters the alveoli, diffuses across the alveolar membrane, and enters the bloodstream. Once in the bloodstream nicotine travels to the brain and then is delivered...