Here are some facts about nicotine:
http://www.nida.nih.gov/MOM/NIC/MOMNIC1.html
"For centuries, people have chewed and smoked tobacco, which comes from the plant nicotiana tabacum. 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.
So why do people smoke? Because nicotine acts in the brain where it can stimulate feelings of pleasure.
Nicotine also activates areas of the brain that are involved in producing feelings of pleasure and reward. Recently, scientists discovered that nicotine raises the levels of a neurotransmitter called dopamine in the parts of the brain that produce feelings of pleasure and reward. Dopamine, which is sometimes called the pleasure molecule, is the same neurotransmitter that is involved in addictions to other drugs such as cocaine and heroin. Researchers now believe that this change in dopamine may play a key role in all addictions. ..
Did you know that nicotine is as addictive as heroin or cocaine? If someone uses nicotine again and again...his or her body develops a tolerance for it. Eventually, a person can become addicted. Once a person becomes addicted, it is extremely difficult to quit. People who start smoking before the age of 21 have the hardest time quitting...
When nicotine addicts stop smoking they may suffer from restlessness, hunger, depression, headaches, and other uncomfortable feelings. These are called "withdrawal symptoms" because they happen when nicotine is withdrawn from the body."