Beginning in the early 2000s, the results of various studies which examined lifestyle choices as possible factors in the development of Parkinson's diseaseplugin-autotooltip__plain plugin-autotooltip_bigParkinson's disease

Also known as Parkinson disease, Parkinson's, idiopathic Parkinsonism, primary Parkinsonism, PD, or paralysis agitans - is a degenerative disorder of the central nervous system resulting from the death of dopamine-containing cells in the (PD) began suggesting a strong link with long-term smoking.

A 2016 'review study' which compiled and compared results from groups of earlier research papers, was published in The Lancet, Neurology (Volume 15, Issue 12, pp. 1257-1272). It found that :

Overall, the results suggest that smoking reduced Parkinson’s disease risk by 74%."

Source : The epidemiology of Parkinson's disease: risk factors and prevention^{[ paywalled ]}

There is currently no agreed explanation for this very dramatic effect.

In an effort to understand the connection, current research is primarily focusing on nicotine itself - including the use of transdermal patches. It's suggested that its effects may be correlated with its activation of nicotinic acetylcholine receptors in the brain. (ref.)

But there could be other factors, e.g. socio-economic etc.. There are also suggestions that smoking might change access to various chemical elements, such as magnesium (Mg) :

Even if the mechanism of protection against PD mediated by nicotinism remains unexplained, Mg deficit might serve as one of the factors promoting/keeping the habit."

Source ref,

Notes :

1) The effects of smoking appear to be both dose-dependent and duration-dependent - i.e. the more smoking, the stronger the effect (ref.)

2) Smoking reduces the risk of developing PD, but does not help those patients who are already affected.

Editor's note :In case this article may seem to encourage smoking, it's worth pointing out that it's been known since the 1930s that regular smoking has a very wide range of dangerous side effects and serious disease risks. Much of the evidence was suppressed by the tobacco companies for decades - with the assistance of some major governments, who were profiting from tobacco taxes and early death rates, which meant lower pension payouts.

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