Health benefits of smoking
Some studies have discovered health benefits correlated with smoking. These studies observed a reduction in the occurrence of some diseases, but all such studies stressed that the benefits of smoking did not outweigh the risks.
Several types of "Smoker’s Paradoxes", i.e. cases where smoking appears to have specific beneficial effects, have been observed; often the actual mechanism remains undetermined. For instance, recent studies suggest that smokers require less frequent repeated revascularization after percutaneous coronary intervention (PCI). Risk of ulcerative colitis has been frequently shown to be reduced by smokers on a dose-dependent basis; the effect is eliminated if the individual stops smoking. Smoking appears to interfere with development of Kaposi's sarcoma, breast cancer among women carrying the very high risk BRCA gene, preeclampsia, and atopic disorders such as allergic asthma. A plausible mechanism of action in these cases may be the nicotine in tobacco smoke acting as an anti-inflammatory agent and interfering with the disease process.
In mice, studies have shown nicotine can reduce the amount of DOI-induced head twitches (meant to model tics) related to Tourette's Syndrome.
A large body of evidence suggests that the risks of neurological diseases such as Parkinson's disease or Alzheimer's disease might be twice as high for non-smokers than for smokers. Many such papers regarding Alzheimer's disease and Parkinson's Disease have been published. A plausible explanation for these cases may be the effect of nicotine, a cholinergic stimulant, decreasing the levels of acetylcholine in the smoker's brain; Parkinson's disease occurs when the effect of dopamine is less than that of acetylcholine. Opponents counter by noting that consumption of pure nicotine may be as beneficial as smoking without the risk.
Other Alzheimer's studies, however, challenge these 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.
Though the negative correlation between smoking and Parkinson's disease is recognized, the causality has not been established. The relationship may be artifact (observational) based on clusters of behavioral and personality differences in the pre-Parkinsonian population versus the smoking population.
Considering the high rates of physical sickness and deaths  among persons suffering from schizophrenia, one of smoking's short term benefits is its temporary effect to improve alertness and cognitive functioning in that disease. It has been postulated that the mechanism of this effect is that schizophrenics have a disturbance of nicotinic receptor functioning.