Lead researcher Dr. Gareth Hagger-Johnson of the University College London said smoking and heavier alcohol consumption often occur at the same time, and their combined effect on cognition might be larger than the sum of their individual effects.
The research team assessed 6,473 adults -- 4,635 men and 1,838 women -- ages 45-69 over a 10-year period, who participated in the Whitehall II study of British civil servants.
All participants were asked about their cigarette and alcohol consumption, and their cognitive function including verbal and mathematical reasoning, short-term verbal memory and verbal fluency was then assessed three times over the 10-year period.
However, among smokers, cognitive decline was found to be faster as the number of alcohol units consumed increased.
"Our research shows that cognitive decline was 36 percent faster in those people who reported both cigarette smoking and drinking alcohol above the recommended limits -- 14 units per week for women, 21 units per week for men. When we looked at people who were heavy-drinking smokers, we found that for every 10 years that they aged their brains aged the equivalent of 12 years."
The study, published in the British Journal of Psychiatry, suggested people should be advised not to combine smoking and drinking -- particularly from mid-life onwards. "Healthy behaviors in midlife may prevent cognitive decline into early old age," the researchers said.