There is convincing evidence of an association between smoking and reduced risk for PD, possibly mediated by an effect on nicotinic receptors. Nicotinic receptors are involved in learning and memory, and smoking may therefore theoretically protect against cognitive decline and dementia in PD. However, smoking also increases inflammation and oxidative stress and is associated with cardiovascular disease. Longitudinal studies have supported the hypothesis that smoking may reduce the risk for dementia and cognitive decline  in PD. However, another longitudinal study did not find an association between smoking and cognitive impairment .
Medications and cholinergic deficits
Patients with cholinergic deficits have a higher risk of developing dementia. It has been shown that cholinergic dysfunction in the cerebral cortex was significantly higher in PD-D patients than in a PD cohort without cognitive dysfunction [62, 67, 85-89]. Thus it is not surprising that anticholinergic drugs, which are traditionally used in PD, are associated with cognitive decline . Autopsy studies in PD patients who were treated with anticholinergic medications for a long time revealed that amyloid plaque densities were more than 2.5 times higher than in PD patients who had no or limited anticholinergic treatment [88, 91]. Another drug used in PD, amantadine, was found to decrease the risk of dementia in a retrospective, naturalistic longitudinal study ; this finding, however, needs confirmation. Dopaminergic antiparkinsonian drugs may affect cognition in a complex way, but a significant association with risk for dementia has not been convincingly demonstrated.
The incidence of PD-D has been shown to be higher in men than in women, particularly in elderly patients .
Risk factors common for AD and vascular dementia, such as high cholesterol , head trauma, diabetes mellitus, and hypertension, were not associated with risk for PD-D [1, 33]. There is also conflicting evidence regarding the association of hyperhomocysteinaemia, a well-known risk factor for cognitive decline in the general population, with the risk for PD-D. Some , but not all , studies have found such an association.