Most smokers use tobacco regularly because they are addicted to nicotine. Addiction is characterized by compulsive drug-seeking and use, even in the face of negative health consequences.
A surge of endorphins in the reward circuits of the brain causes a slight, brief euphoria when nicotine is administered. This surge is much briefer than the “high” associated with other drugs. However, like other drugs of abuse, nicotine increases the levels of dopamine in these reward circuits, which reinforces the behavior of taking the drug.
Repeated use leads to changes in other brain circuits involved in learning, stress, and self-control. For many tobacco users, these long-term brain changes result in addiction, which involves withdrawal symptoms when not smoking, and difficulty adhering to the resolution to quit.
The way nicotine is processed by the body also contributes to its addictiveness. When cigarette smoke enters the lungs, nicotine is absorbed rapidly in the blood and delivered quickly to the brain, so that nicotine levels peak within 10 seconds of inhalation. But the acute effects of nicotine also dissipate quickly, along with the associated feelings of reward; this rapid cycle causes the smoker to continue using to maintain the pleasurable effects and prevent withdrawal symptoms.