A new study published May 20, 2026, in Neurology^® found that people who quit smoking had a lower risk of developing dementia compared to those who continued smoking—but the amount of weight people gained after quitting made a difference. 

Researchers followed more than 32,800 middle-aged and older adults who did not have dementia at the start of the study. Participants were followed for about 10 years and interviewed every two years about their smoking habits, body weight, and overall health. During the study, nearly 5,900 people developed dementia. 

After accounting for factors such as age, physical activity, and heart health, researchers found that people who quit smoking during the follow-up period had a 16% lower risk of dementia compared to people who were currently smoking. The benefit of quitting grew over time, and after about seven years, the dementia risk for people who quit was close to that of people who had never smoked. 

However, weight gain appeared to affect these benefits. People who quit smoking and gained little to moderate weight (about 11 pounds or less) still had a lower risk of dementia and slower decline in thinking skills. However, people who had quit smoking but gained 22 pounds or more did not experience a reduced risk of dementia.

“People often worry about what happens after they quit smoking—including weight gain,” says study author Hui Chen, PhD. “What we found is that quitting is still associated with better brain outcomes, but maintaining your weight may help preserve those benefits.”

This study does not prove that quitting smoking directly lowers dementia risk. The findings also rely on people reporting their own smoking habits and body weight, which may not always be exact.

Key Takeaways

• In this study, quitting smoking was linked to a lower risk of dementia compared to people who continued to smoke.
• People who quit smoking and gained about 11 pounds or less still had a lower risk of dementia and slower decline in thinking skills.
• People who quit smoking but gained about 22 pounds or more did not have the reduced risk of dementia.

Medically reviewed by Sarah Song, MD, MPH, FAAN