A new study suggests smoking affects not only the smoker's brain, but his mind as well.

Researchers at the University College London found that middle-aged men who smoked showed a greater decline in mental function than those who had never smoked.

"Our results show that the association between smoking and cognition, particularly at older ages, is likely to be underestimated owing to higher risk of death and dropout among smokers," the researchers wrote.

Smoking actually seemed to speed up the cognitive aging process by about 10 years.

"This study underscores that smoking is bad for your brain," Marc Gordon, chief of neurology at Zucker Hillside Hospital in Glen Oaks, New York, who was not involved in the study told the Agence France-Presse.

"Mid-life smoking is a modifiable risk factor with an effect size roughly equivalent to 10 years of aging on the rate of cognitive decline," he added.

The researchers say the effects appear to be reversable. Results showed men who quit smoking more than 10 years before the tests were performed did as well as men who had never smoked.

The study, published the Archives of General Psychiatry, followed more than 5,000 men and 2,100 women in the British civil service. Research subjects entered the study at an average age of 56 and were followed for up to 25 years.