It has been chewed, sniffed, and smoked. Today it is most commonly used in the form of cigarettes. Cigarette smoke contains thousands of chemicals, many are responsible for its powerful addictiveness.
Nicotine acts as a stimulant on the central nervous system.
Nicotine has many metabolic affects. It increases the metabolic rate, raises blood pressure, changes muscle tension and affects certain brain chemicals and hormones. These and other metabolic changes create a pleasurable sensation in the smoker that is felt as relaxation. Another factor is that the 'tolerance' to the effects of nicotine develops very quickly. Tolerance is a word that kind of means the smoker wants to smoke more and needs to smoke more.