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Ulcerative colitis is twice as common in non-smokers as in smokers. The nicotine in tobacco smoke acts as an anti-inflammatory agent.
Also, it is thought that the risks of Parkinson's disease or Alzheimer's disease may be significantly lower for smokers than for non-smokers though this idea has not been definitively proven. According to previous epidemiological studies, cigarette smokers are 50% less likely to have PD or AD than are age- and gender-matched nonsmokers. One explanation given for this effect is the fact that nicotine allegedly increases the levels of acetylcholine in the smoker's brain. Parkinson's disease consequently occurs when the effect of dopamine is less than that of acetylcholine. Another explanation may simply be that smokers die before they reach the age at which the onset of Parkinson's disease occurs. However, nicotine, a cholinergic stimulant, is the only substance in tobacco smoke which has those positive effects, and the consumption of pure nicotine would likely be more beneficial than smoking.
More recent evidence has challenged the earlier epidemiological studies on methodological grounds. A prospective Rotterdam Study found that the incidence of Alzheimer's disease is more than double for smokers as compared to non-smokers and the Honolulu Heart Program (a longitudinal cohort study) also found more than twice the risk for Alzheimer's disease among medium and heavy smokers as compared to non-smokers."