According to researchers at King’s College London, smoking “rots” the brain by damaging memory, learning and reasoning.
A study of 8,800 people over 50 showed high blood pressure and being overweight also seemed to affect the brain, but to a lesser extent.
Scientists involved said people needed to be aware that lifestyles could damage the mind as well as the body.
Their study was published in the journal Age and Ageing.
Researchers at King’s College London were investigating links between the likelihood of a heart attack or stroke and the state of the brain.
Data about the health and lifestyle of a group of over-50s was collected and brain tests, such as making participants learn new words or name as many animals as they could in a minute, were also performed.
They were all tested again after four and then eight years.
The results showed that the overall risk of a heart attack or stroke was “significantly associated with cognitive decline” with those at the highest risk showing the greatest decline.
It also said there was a “consistent association” between smoking and lower scores in the tests.
One of the researchers, Dr. Alex Dregan, said: “Cognitive decline becomes more common with ageing and for an increasing number of people interferes with daily functioning and well-being.
“We have identified a number of risk factors which could be associated with accelerated cognitive decline, all of which, could be modifiable.”
He added: “We need to make people aware of the need to do some lifestyle changes because of the risk of cognitive decline.”
The researchers do not know how such a decline could affect people going about their daily life. They are also unsure whether the early drop in brain function could lead to conditions such as dementia.