Cigarettes contain nicotine, a psychoactive or mood altering drug. When a person smokes, nicotine reaches the brain in eight seconds and causes the release of a chemical called dopamine. Dopamine causes feelings of pleasure and relaxation, a sensation the body craves again and again. However, during these feelings of perceived relaxation, the body is actually experiencing increased stress (blood pressure and heart rate increase), muscles become tense, and less oxygen is available to the body and brain. The feelings of relief described by people who smoke may just be relief from short-term symptoms of withdrawal from nicotine.