



The increased risk of dementia was more muted in less-heavy smokers: Those who smoked one to two packs a day had a 44% greater risk of developing dementia compared with nonsmokers, while those who smoked a half to one pack a day had a 37% increased risk.

A more than two-pack-a-day smoking habit in middle age doubles the risk of dementia. At one to two packs a day, the risk is 44% greater. Getty Images

About 25% of the sample developed dementia in older age, according to the results published in the journal Archives of Internal Medicine.

"It's a pretty clear picture that heavy smoking … elevates your risk of dementia," said Rachel Whitmer, a study author and scientist at the research division of Kaiser Permanente in Oakland, Calif. "If you are a heavy smoker and you're lucky enough to make it to old age, you're not in the clear. You're still at risk for dementia."

The researchers say they think smoking's role in the development of Alzheimer's and vascular dementia may include possible negative effects on brain blood vessels and brain cells.

"We know smoking compromises the vascular system by affecting blood pressure and elevates blood-clotting factors, and we know vascular health plays a role in risk of Alzheimer's disease," Dr. Whitmer says.

Previous studies had raised questions about whether smoking provided a protective effect. Some research in the early 1990s indicated that smoking reduced the risk of cognitive impairment and dementia, and a recent study showed that smoking appears to improve current cognitive performance, probably because of the effects of the stimulant nicotine.

There also is evidence to suggest that smokers are less likely to get Parkinson's disease. Why this would be the case is "a big question mark," but would likely involve how nicotine affects brain chemicals such as dopamine, according to Miia Kivipelto, deputy head of the Aging Research Center at the Karolinska Institute in Stockholm, Sweden.

Earlier studies suggesting protective effects of smoking were biased, due to the likelihood that a higher proportion of heavy smokers died before developing Alzheimer's disease, said Lenore Launer, chief of the neuro-epidemiology section at the National Institute of Aging who conducted some of the previous research but wasn't involved in the current study.

Several other large-scale studies that followed patients over time have shown that smoking is a risk factor for dementia.

In the current study, researchers asked patients, who were 50 to 60 years old at the start of the study, whether they smoked or not—unlike previous studies of individuals in their 70s and 80s, who were asked to recall health habits from years ago. People often incorrectly remember information from years ago, or they may already be suffering from some type of mild cognitive impairment, Dr. Whitmer said.

Significantly, former smokers—those who'd reported having quit by the start of the study—didn't have an increased risk of dementia. That finding dovetails with other work to suggest "that if you quit smoking after seven to 10 years, especially at a young age, there is a lot of evidence you could be OK," Dr. Whitmer said.

"The silver lining is that this risk factor is completely modifiable," says William Thies, chief medical and scientific officer of the Alzheimer's Association. "For anyone who is contemplating ending a smoking habit, this study's findings provide yet another excellent reason to do so."

In the recent study, the length of time that people smoked, and how long ago they'd quit, couldn't be determined.

Dr. Whitmer is currently conducting a study to examine the long-term risk for dementia for people who began smoking in young adulthood.

"The brain is not immune to the long-term consequences of smoking," she said.

Write to Shirley S. Wang at