Background While there is a suggestion that self-reported tobacco smoking may be a risk factor for dementia, to date, it has not been possible to explore the thresholds at which this exposure elevates risk. Accordingly, our aim was to relate cotinine, a biomarker of tobacco smoking, to risk of dementia death.

Methods We pooled 14 prospective cohort studies that held data on cotinine (plasma or saliva), covariates and death records.

Results In the 33 032 study members (17 107 women) with salivary cotinine data, a mean duration of 8.3 years of follow-up gave rise to 135 deaths ascribed to dementia; while in 15 130 study members (7995 women) with plasma cotinine data, there were 119 dementia deaths during 14.3 years of mortality surveillance. After multiple adjustment, both plasma cotinine (per 1 SD higher cotinine; 95% CI 1.29; (1.05 to 1.59)) and salivary cotinine (1.10 (0.89 to 1.36)) were positively related to dementia risk, with stronger effects apparent for plasma.

Conclusion Our finding that plasma cotinine was related to an elevated risk of dementia death warrants testing in studies with measures of disease onset as opposed to just mortality.