Smoking, of course, damages the lungs and blood vessels, and contributes to an array of health problems, but nicotine — the calming chemical that cigarettes deliver — might actually be good for the aging brain.

Smokers, for example, are less likely to develop Alzheimer's disease — a phenomenon that has long puzzled scientists because smoking contributes to cardiovascular disease, which strongly increases the risk of Alzheimer's.

But closer investigation revealed that smoking doesn't confer the protection; nicotine does.

A study of Alzheimer's patients showed that those who wore nicotine patches were better able to remember and pay attention than those who didn't. Another study showed that nicotine boosted cognitive function in older people who didn't have Alzheimer's, but were showing signs of age-related mental decline.

Nicotine also seems to protect against Parkinson's disease, in which the death of cells in a small area of the brain results in tremors, impairing movement and as well as cognitive difficulties.

So what's going on? How does the dreaded addictive component of cigarettes produce health benefits?

For starters, nicotine by itself isn't very addictive at all, according to Dr. Paul Newhouse, the director of Vanderbilt University's Center for Cognitive Medicine. Nicotine seems to require assistance from other substances found in tobacco to get people hooked.

"People won't smoke without nicotine in cigarettes, but they won't take nicotine by itself," said Newhouse, who has done extensive research into beneficial effects of nicotine on the brain. "Nicotine is not reinforcing enough. That's why FDA agreed nicotine could be sold over the counter. No one wants to take it because it's not pleasant enough by itself. And it's hard to get animals to self-administer nicotine the way they will with cocaine."

Nicotine is chemically similar to acetylcholine, a neurotransmitter in the brain that declines in Alzheimer's disease. Drugs such as Aricept help people with Alzheimer's by boosting brain levels of acetylcholine. Apparently, nicotine binds to the receptors in the brain normally occupied by acetylcholine, which benefits people who need more, but it has no apparent effect on those who don't.

"Nicotine doesn't appear to enhance normal people," Newhouse said, "but in people who show some degree of cognitive impairment, nicotine appears to produce a modest but measurable effect on cognitive function, particularly in areas of attention and, to some extent, memory."

Newhouse and his colleagues are testing nicotine to see if it improves other cognitive problems like the mental fogginess known as "chemo brain" that afflicts cancer patients undergoing chemotherapy. They've also started a study of adults with Down syndrome, who almost always develop Alzheimer's disease by the time they reach middle age. Even people with HIV, which appears to cause accelerated cognitive decline, may benefit.

What makes nicotine especially attractive as a treatment is the fact it causes virtually no side effects, according to Newhouse.

"It seems very safe even in nonsmokers," he said. "In our studies we find it actually reduces blood pressure chronically. And there were no addiction or withdrawal problems, and nobody started smoking cigarettes. The risk of addiction to nicotine alone is virtually nil."

Tom Valeo writes on health matters. He can be reached at tom.valeo@gmail.com.