Roughly 40 percent of Americans have tried marijuana, and about 10 percent continue to use it on a regular basis, so this issue is probably more common than one might expect.

The active chemical in marijuana, THC, works on receptors in the brain. In the short term, or within hours of smoking the drug, lower doses can cause feelings of relaxation or distort your senses, whereas higher doses may create hallucinations, learning and memory problems and impaired coordination -- which is why it's not a good idea to drive or play sports when under the influence.

Over the long term, the tar in marijuana may put a person at risk for lung cancer, much as cigarette smoke does. Smoking marijuana can also cause asthma, bronchitis, sinus infections and a sore throat, especially in heavy users. Marijuana may also impair the immune system's ability to fight off certain infections. There is also some evidence that marijuana can trigger heart attacks in people who already have or are at risk for coronary heart disease.

Because marijuana acts on the brain, chronic use can also make it difficult to concentrate, pay attention and learn. Some studies have shown that regular marijuana smokers may function at a lower intellectual level all the time since the drug can keep affecting the brain for weeks after using.

People who try to stop using marijuana after years of use may have withdrawal symptoms such as anxiety, irritability, drug craving and insomnia, making it hard to quit. Because of the possible difficulty quitting the drug and the many health concerns associated with long-term smoking, I advise regular users to consult with their doctor or an addiction medicine specialist to help them stop.

He promised to obey the no-smoking rules in the White House, but whether that meant he would be ducking out the back door for a smoke is not known. His transition team declined to answer any questions about his smoking, past or present, or his efforts to quit.

Antismoking activists would love to see him use his bully pulpit to inspire others to join him in trying to kick the habit, but he has not yet taken up their cause.

The last president to smoke more than occasionally was Gerald R. Ford, who was quite fond of his pipes. Jimmy Carter and both Presidents George Bush were reportedly abstainers, but Bill Clinton liked cigars from time to time, though he may have chewed more than he smoked.

Mr. Obama’s heaviest smoking was seven or eight cigarettes a day, but three was more typical, according to an interview published in the November issue of Men’s Health magazine. In a letter given to reporters before the election, Mr. Obama’s doctor described his smoking history as “intermittent,” and said he had quit several times and was using Nicorette gum, a form of nicotine replacement, “with success.” Mr. Obama was often seen chewing gum during the campaign.

His pattern matches that of millions of other people who have resolved but stumbled in their efforts to give up cigarettes. Today, 21 percent of Americans smoke, down from 28 percent in 1988. Off-again-on-again smoking and serial quitting are common, as is the long-term use of nicotine gum and patches.

“It takes the average smoker 8 to 10 times before he is able to quit successfully,” said Dr. Steven A. Schroeder, director of the Smoking Cessation Leadership Center at the University of California, San Francisco.

Dr. Schroeder said that counseling was helpful, and that if Mr. Obama were his patient, he would urge him to try it, even if only by telephone, toll free at 1-800-QUITNOW (1-800-784-8669). With nicotine replacements and counseling, quit rates at one year are 15 percent to 30 percent, Dr. Schroeder said, about twice that of those who try without help.