It's widely known that smoking ages your skin. But now, research has shed a light on how it ages the brain.
Smoking has been found to accelerate brain ageing and impair thinking skills such as planning, decision-making and problem-solving.
But scientists suggest that the harmful effects of it can be reversed in those who give up the habit.
Researchers analysed magnetic resonance imaging (MRI) brain scan data on 504 men and women with an average age of 73. Roughly half of the participants were former or current smokers, while the rest had never smoked.
The findings showed that smoking increased the rate at which the outer layer of the brain thins with age. This part of the brain, called the cerebral cortex, is linked to many higher functions and plays a key role in memory, attention, language and awareness.
Lead scientist Professor Ian Deary, director of the centre for cognitive ageing at the University of Edinburgh, said: "It is important to know what is associated with brain health in older age and our study shows that the rate of smoking-related thinning to the brain is approximately twice the rate observed in previous, smaller studies.
"However, at the same time, our study also suggests that stopping smoking might allow the brain's cortex to recover some of its thickness, though we need to conduct further studies to test this."
The research, published in the journal Molecular Psychiatry, forms part of The Disconnected Mind - a larger project investigating brain ageing funded by Age UK.
Professor James Goodwin, chief scientist at Age UK, said: "We all know smoking is bad for our lungs and heart, but it's important we also understand just how bad it is for our brain.
"This study shows how smoking speeds up the decline of the important thinking skills we rely on - in a sense accelerating brain ageing - in addition to increasing the risk of dementia and many other illnesses.
"While avoiding smoking is the best way to reduce the risk of brain decline, dementia and other cognitive diseases, this study gives new hope that quitting smoking, even in mid-life, can bring important benefits to the brain, as well as the rest of the body."
He added: "With research suggesting that older people's fear of developing dementia outweighs that of cancer, it is important we inform people about the simple steps they can take to safeguard against this horrible and distressing disease.
"Brain decline is not an inevitable part of ageing, it is something we can protect ourselves against by making changes to our lifestyle - with avoiding smoking being one of them."