A medication being tested to help smokers kick the habit also may help avoid the weight gain that is common after quitting—but only in women, according to a study published in the December issue of Biological Psychiatry.

Naltrexone, an opioid blocker that can dampen the desire for alcohol, heroin, and nicotine, as well as the pleasures of eating, helped men quit smoking. It improved their quit rates after three months of treatment in a controlled trial, from 17 percent for those who didn't get the drug up to 30 percent for those who did.

The drug didn't improve quit rates for women beyond that of the placebo. But for the women who successfully quit smoking, their weight gain was reduced by more than half. After three months, those who took naltrexone gained an average of 2.3 pounds while those who took a placebo gained 5.1 pounds.

"When trying to stop smoking, women tend to gain more weight than men and to be more concerned about gaining that weight," says study author Dr. Andrea King, professor of psychiatry and behavioral neuroscience at the University of Chicago Medicine. "Women who try to quit may be so worried about putting on weight in the process that they soon give up, and this is less commonly found in men. Adding naltrexone to standard treatment might help women get through that difficult early period."

While tobacco use is the leading cause of preventable death in the United States, obesity is the second leading cause. Ironically, more than 80 percent of those who are able to stop smoking put on at least five pounds in the year after quitting. Up to 25 percent of those who quit gain more than 15 pounds.

For this study, researchers combined data from the two largest trials using naltrexone to help volunteers stop smoking. The trial included 700 participants, 315 from the University of Chicago and 385 from Yale University. For six to 12 weeks after their quit date, participants took either naltrexone or a placebo. They also used a nicotine patch and attended smoking-cessation counseling for the first month.

After six months, 159 of the 700 participants (23 percent), including 77 women, remained verifiably smoke free. After 12 months, that number fell to 115 (16 percent), including 57 women.

During the first 12 months after quitting, weight gain continued to increase for successful quitters, but the benefits of the early treatment with naltrexone persisted. The effect was diminished, however. From a 50-percent reduction in weight gain for those taking the drug when measured at three months, the difference decreased to 40 percent (7.3 versus 12.1 pounds) at six months, and to 20 percent (13 versus 16.3 pounds) at 12 months.