While smoking has been clearly identified as the cause of many diseases and other health problems, as described above, it has also been observed to reduce the incidence of some diseases (endometrial cancer, Parkinson's disease, ulcerative colitis, hypertension of pregnancy(4) and Alzheimer's disease(147,148)). This is known as a 'protective effect'.
There is evidence to suggest that smoking reduces the risk of cancer of the endometrium (membrane lining of the uterus) in post-menopausal women by an estimated 30% in current smokers.(149) This may be because of the effects of smoking on oestrogen levels. Smoking is known to affect other oestrogen related phenomena: for example, both a woman's age at menopause and her bone density after menopause are related to her oestrogen levels. The action of cigarette smoking is opposite to that of oestrogens in each instance: relative to other women, smokers have an earlier age at menopause and bones that are more brittle. The risk of endometrial cancer is substantially increased by the use of drugs containing oestrogen. Therefore it is not inconsistent that tobacco smoke could counter the effect of oestrogens and reduce the risk of endometrial cancer.(150)
However if some degree of protection from endometrial cancer is afforded by cigarette smoking, it fades to insignificance when compared to the numbers of lives lost through smoking. Some 30 times as many lives are lost due to smoking than saved through lowered incidence of endometrial cancer.(150) Researchers have acknowledged this: 'The present findings [about endometrial cancer] do not have direct public health importance since cigarettes, overall, have serious deleterious effects. However, if these results are confirmed, elucidation of the underlying mechanism whereby smoking reduces the risk would be of interest and might be useful in the development of strategies for preventing endometrial cancer.'(149)
It is estimated that the overall aetiological fraction of endometrial cancer prevented in ex- and current-smokers is -0.10. This means that for every 100 cases of endometrial cancer observed in a community where smoking occurs, if smoking were absent from this community, 10 more would occur.
Tobacco also appears to have a protective effect against Parkinson's disease and hypertension of pregnancy. The overall aetiological fractions for the effects of smoking are -0.35 for Parkinson's disease in men and -0.22 in women. In women, the aetiological fraction for hypertension of pregnancy is -0.07.
The effects of smoking on ulcerative colitis are unclear. In their review, English et al conclude that there is limited evidence that current smoking protects against ulcerative colitis and that there is also limited evidence that past smoking causes ulcerative colitis. The overall effect of ever smoking is that 8% of ulcerative colitis in men and 3% in women is caused by ever smoking.
Recent research indicates that smoking may also protect against the development of Alzheimer's disease in some measure. However, as with the other conditions listed above, tobacco use is not recommended as a prophylactic: 'Although the association is compatible with a protective effect of smoking for familial Alzheimer's disease, it has no relevance for prevention of Alzheimer's disease because of the adverse health effects of smoking.'(148)