Kicking cigarettes later in life may help the brain, too. A new analysis of long-running US health data found that quitting smoking was linked to a lower risk of dementia, but the benefit was much weaker in people who gained substantial weight after they stopped.

The research looked at self-reported health data from more than 32,000 dementia-free retired adults in the United States, around age 60. Participants reported their smoking status and body weight once every two years, and researchers tracked them for 25 years. Over that time, nearly 6,000 cases of dementia were documented.

Compared to current smokers, people who quit smoking during the study had a 16 percent lower risk of dementia. People who had never smoked had a 25 percent lower risk than current smokers.

The analysis also found that people who quit smoking had a similar dementia risk in older age to those who had never smoked at all.

But that pattern came with a limit. The brain benefits were largely seen in participants who did not gain weight or who gained less than 5 kilograms within two years of quitting. If someone quit smoking and gained more than 10 kilograms, there was no longer a significant association between quitting and dementia risk.

Cognitive testing over time also showed slower cognitive decline in people who quit smoking, especially those with only minor weight gain. Quitters who gained little to no weight had roughly 20 percent slower rates of cognitive decline than people who kept smoking.

The researchers said this was gradual, not sudden. The longer a person went without smoking, the greater the benefit to their pace of cognitive decline on certain tests.

Lead researcher Hui Chen, from the Zhejiang University School of Medicine in China, said weight changes after quitting deserve attention.

“People often worry about what happens after they quit smoking, including weight gain and associated metabolic changes,” Chen said in a news release.

“What we found is that quitting is still associated with better brain outcomes, but maintaining your weight may help preserve those benefits.”

The study cannot show how smoking directly affects brain health, and the authors said the findings should be interpreted cautiously.

Smoking is considered one of the leading risk factors for dementia. Some estimates suggest it can raise the odds of developing dementia by 30 to 50 percent. But the article notes those figures largely come from population studies, which can miss individual differences.

The study also points to a problem that often follows smoking cessation. Weight gain is common after quitting, usually less than 5 kilograms in the years that follow. But about 10 to 20 percent of people who quit smoking gain more than 10 kilograms.

Obesity and type 2 diabetes are also linked to dementia risk. The article says that if quitting smoking leads to weight gain, that may raise the risk of diabetes, which in turn raises dementia risk.

Chen said the findings suggest health messaging should also focus on weight management after people stop smoking.

“Our findings suggest that quitting smoking may support long‑term brain health, but they also highlight that what happens after quitting matters,” Chen said.

“Future research is needed to better understand how weight management and other lifestyle factors can help people maximize the cognitive benefits of quitting smoking as they age.”

The study is published in Neurology.

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