Nicotine, which mimics the affects of acetylcholine, acts primarily on the autonomic nervous system. In a dose of less than 50 mg, it can cause respiratory failure and general paralysis. Smaller toxic doses can cause heart palpitations, lowered blood pressure, nausea, and dizziness. A person who smokes inhales approximately 3 mg from one cigarette. This amount increases the heart rate, constricts the blood vessels, and acts on the central nervous system, imparting a feeling of alertness and well-being. Although not considered carcinogenic, nicotine probably contributes to the increased incidence of heart disease seen in smokers and may enhance the growth of tumors caused by carcinogens.

People who use tobacco products develop a physiological addiction to nicotine. Research has shown that nicotine increases the flow of the neurotransmitter dopamine in the brain, creating pleasurable feelings and a craving to keep in the bloodstream levels of nicotine that will maintain these feelings. Lack of nicotine causes withdrawal symptoms (heart rate and blood pressure changes, sleeping problems, brain wave disturbances, and anxiety) in smokers.

Nicotine-containing chewing gums and skin patches that administer nicotine to people who are trying to cease smoking have been developed. Although the rate of absorption is slower with these methods than with smoking—smoking delivers nicotine to the brain within six seconds—and although nicotine obtained in this way does not provide the same pleasurable results as smoking, the gums and patches do help relieve some of the symptoms of withdrawal. Combining the use of patches or gum with continued smoking can result in nicotine overdose and toxicity, causing nausea, palpitations, and headache. Nicotine nasal sprays and inhalers more closely mimic the delivery and intensity of nicotine obtained by smoking. Some researchers have suggested, however, that prolonged use of nicotine replacement, especially inhalers, beyond the few months recommended to break the cigarette habit could damage cells lining the blood vessels and lungs.

See also smoking.