Smoking cigarettes is very protecting against parkinsons. The evidence is clear, large and replicated in large samples. Hypothetically, would someone with strong genetic indications of risk for parkinsons, and genetic indications that they are protective against cardiovascular disease, lung cancer and that kind of other smoking related diseases be making a healthy choice to start smoking?.
SSC describes a related affect and also mentions parkinsons in SCHIZOPHRENIA: NO SMOKING GUN (recent).
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