University of Minnesota researchers say 77 people participated in the double-blind, placebo controlled study. Fifty-eight men and women took 50, 100 or 150 milligrams of naltrexone every day for 18 weeks. Forty percent of the 49 participants who took the drug and completed the study quit gambling for at least one month.

The study, published in the Journal of Clinical Psychiatry, says the subjects' urge to gamble also significantly dropped in intensity and frequency.

The other 19 participants took a placebo. About 10 percent of those who took the placebo were able to abstain from gambling. Study participants were ages 18 to 75 and reported gambling for six to 32 hours each week.

"This is good news for people who have a gambling problem," Dr. Jon Grant of the University of Minnesota said in a statement. "This is the first time people have a proven medication that can help them get their behavior under control."

While the drug is not a cure for gambling, Grant says it offers hope to problem gamblers.