Nicotine reaches the brain within 10 seconds after smoke enters the lungs and affects the body. Nicotine raises the heart and respiration (breathing) rates. Nicotine also causes more glucose (blood sugar) to be released into the blood, which may explain why smokers say they feel more alert after smoking. Nicotine also causes the brain cells to release an unusually large amount of a chemical called dopamine. Dopamine stimulates pleasure centers in the brain, making the smoker feel good.
The effects of nicotine do not last very long. When the effects wear off, the smoker feels a strong urge to smoke again to get more nicotine. Repeated doses of nicotine alter the brain's activities. The brain reduces the amount of dopamine that it produces. The number of receptors that carry dopamine to the cells is also reduced. When this happens, the smoker needs nicotine just to have normal levels of dopamine in the brain. If the level of dopamine drops, the smoker feels irritable and depressed.
Both young and older smokers can become addicted to nicotine. In adults, nicotine addiction is linked to the amount and frequency of tobacco used. In teens, nicotine addiction appears to be linked to the length of time they have been regular tobacco users. Teens who only smoke small amounts but who smoke daily are still at high risk of becoming addicted to nicotine.