Identifying modifiable risk factors for cognitive decline may inform prevention of dementia.

To examine the combined impact of cigarette smoking and heavy alcohol consumption on cognitive decline from midlife.

Prospective cohort study (Whitehall II cohort) with three clinical examinations in 1997/99, 2002/04 and 2007/09. Participants were 6473 adults (72% men), mean age 55.76 years (s.d. = 6.02) in 1997/99. Four cognitive tests, assessed three times over 10 years, combined into a global z-score (mean 0, s.d. = 1).

Age-related decline in the global cognitive score was faster in individuals who were smoking heavy drinkers than in non-smoking moderate alcohol drinkers (reference group). The interaction term (P = 0.04) suggested that the combined effects of smoking and alcohol consumption were greater than their individual effects. Adjusting for age, gender, education and chronic diseases, 10-year decline in global cognition was −0.42 z-scores (95% Cl −0.45 to −0.39) for the reference group. In individuals who were heavy alcohol drinkers who also smoked the decline was −0.57 z-scores (95% Cl −0.67 to −0.48); 36% faster than the reference group.

Individuals who were smokers who drank alcohol heavily had a 36% faster cognitive decline, equivalent to an age-effect of 2 extra years over 10-year follow-up, compared with individuals who were non-smoking moderate drinkers.