Although tobacco smoking has long been associated with diseases of the lungs and cardiovascular system, numerous studies have demonstrated a negative association between tobacco smoking and ulcerative colitis, and the neurodegenerative diseases, Alzheimer's disease (AD) and Parkinson's disease (PD). The evidence suggests that nicotine--the main pharmacologically active ingredient of tobacco--appears to be responsible for this effect. Pure nicotine has no known carcinogenic properties and can be administered in numerous ways including transdermal patches and tablets. As a therapeutic agent, its association with tobacco can be likened to morphine and opium smoking. There is ample clinical evidence to suggest that nicotine could be beneficial in the treatment of some patients with diseases. Pharmacologically, nicotine acts on cholinergic (nicotinic-specific) receptors which are depleted in AD and PD. Nicotinic receptors also interact closely with several neurotransmitters including dopamine, which is implicated in both PD and Gilles de la Tourettes's syndrome. There is no doubt that tobacco smoking can be harmful and no-one should be encouraged to smoke. However, although nicotine has many harmful side-effects, it may have therapeutic value or at the very least be a useful tool for future drug development.