The reason tobacco is used by so many people is because it contains a powerful drug known as nicotine. When tobacco is smoked, nicotine is absorbed by the lungs and quickly moved into the bloodstream, where it is circulated throughout the brain. All of this happens very rapidly. In fact, nicotine reaches the brain within 8 seconds after someone inhales tobacco smoke. Nicotine affects the entire body.
Nicotine acts directly on the heart to change heart rate and blood pressure.
It also acts on the nerves that control respiration to change breathing patterns. In high concentrations, nicotine is deadly, in fact one drop of purified nicotine on the tongue will kill a person. It's so lethal that it has been used as a pesticide for centuries. Because nicotine acts in the brain where it can stimulate feelings of pleasure.