An alkaloid, which is the major psychoactive substance in tobacco. It has both stimulant and relaxing effects. It produces an alerting effect on the electroencephalogram and, in some individuals, an increased capacity to focus attention. In others, it reduces anxiety and irritability. Nicotine is used in the form of inhaled tobacco smoke, "smokeless tobacco" (such as chewing tobacco), snuff, or nicotine gum. Each puff of inhaled tobacco smoke contains nicotine that is rapidly absorbed through the lungs and delivered to the brain within seconds. Considerable tolerance and dependence develop to nicotine. Because of its rapid metabolism, brain levels of nicotine fall rapidly and the smoker experiences craving for a further cigarette 30-40 minutes after finishing the last one.
In the nicotine user who has become physically dependent, a withdrawal syndrome develops within a few hours of the last dose: craving for a smoke, irritability, anxiety, anger, impaired concentration, increased appetite, decreased heart rate, and sometimes headaches and sleep disturbances. Craving peaks at 24 hours and then declines over a period of several weeks, although it may be evoked by stimuli associated with previous smoking habits. Tobacco products contain many constituents besides nicotine. Sustained use of tobacco products may result in lung, head, or neck cancers, heart disease, chronic bronchitis, emphysema, and other physical disorders. Nicotine dependence (ICD-10 FI7.2) is classed as a tobacco use disorder under the psychoactive substance use disorders in ICD-10.