It is already widely acknowledged that smoking and drinking too much are bad for our health. Now a new study has found that when done together, smoking and heavy drinking speed up cognitive decline.

According to UK scientists, these two habits often occur together, therefore they wanted to assess the combined effect of them on cognition - a group of mental processes that include memory, problem solving, decision making and language skills.

They monitored the progress of over 6,400 adults, aged between 45 and 69, over a 10-year-period. All of the participants provided information about their smoking and drinking habits and their cognitive abilities were tested throughout. This included tests on verbal fluency, memory and mathematical reasoning.

The scientists from University College London found that cognitive decline in current smokers who were also heavy drinkers was 36% faster compared to non-smokers who were moderate drinkers. Furthermore, among smokers, the more alcohol a person consumed, the faster the cognitive decline.

"Our research shows that cognitive decline was 36% faster in those people who reported both cigarette smoking and drinking alcohol above the recommended limits. 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," explained lead researcher, Dr Gareth Hagger-Johnson.

The recommended weekly allowance of alcohol is no more than 21 units for men and 14 units for women. A unit of alcohol is considered a small glass of wine (100ml), a pub measure of spirits (35.5ml) or a glass of beer/cider/stout (284ml).

The scientists pointed out that from a public health perspective, ‘the increasing burden associated with cognitive aging could be reduced if lifestyle factors can be modified'.

"We believe that people should not drink alcohol more heavily in the belief that alcohol is a protective factor against cognitive decline. Current advice is that smokers should stop or cut down, and people should avoid heavy alcohol drinking.

"Our study suggests that people should also be advised not to combine these two unhealthy behaviours - particularly from mid-life onwards. Healthy behaviours in midlife may prevent cognitive decline into early old age," they said.

Details of these findings are published in the British Journal of Psychiatry.