Nicotine is the addictive substance in tobacco and its withdrawal is responsible for a range of unpleasant symptoms after smoking cessation. Although it produces acute physiological effects, nicotine alone is not carcinogenic and does not appear to cause the vascular disease associated with smoking. Nicotine replacement has been shown to be a safe and effective pharmacological treatment for tobacco dependence in certain smokers. Its efficacy is greatest when prescribed for those who are motivated and highly nicotine-dependent. It is probably not indicated for smokers with a low degree of nicotine dependence. Studies of nicotine chewing gum conducted in special referral clinics have generally produced positive results, whereas those conducted in community practice settings have shown a smaller benefit when compared with placebo. When the results of all published placebo-controlled trials are pooled the typical improvement in smoking cessation rate is 40% (odds ratio continued smoking 0.6; 95% confidence interval 0.5-0.71; P less than 0.00001). The best results with nicotine chewing gum have been obtained with multicomponent programmes which have included some counselling and ongoing follow up and support. Early reports of success with a transdermal nicotine preparation suggest that it may have similar efficacy to nicotine gum. Clonidine administered orally or transdermally has also been shown to reduce tobacco withdrawal symptoms but requires more convincing evidence of long-term efficacy before it can be recommended for routine use. Currently available over-the-counter products, apart from nicotine chewing gum, have not been shown to be effective.