Researchers say an injectable version of a drug used to treat alcoholism, combined with therapy, resulted in a reduction of heavy drinking by test subjects over a period of six months.

There are three drugs now available to treat alcoholism. One of these is the drug naltrexone, which is administered as a daily pill.

But Dr. Marc Galanter of the New York University Division of Alcoholism and Drug Abuse says the success of daily doses of naltrexone depends on the commitment of the patient.

"I have used it in pill form with some patients, and for some of them, it's been very helpful. But these are the ones who are very committed to treatment and able to overcome the denial that might impinge on them every day," he says.

A study published this week in the Journal of the American Medical Association involved more than 600 patients at 24 university hospitals in 2002 and 2003. One third of the group was given a placebo. The rest were given varying doses of the injectable form of naltrexone, known as Vivitrex. All the patients were also given therapy. The company that is developing Vivitrex, Alkermes, sponsored the study.

Researchers at the University of North Carolina in Chapel Hill and elsewhere found a decrease in the number of days of heavy drinking from an average 19 days to three over six months.

James Garbutt is the study's lead author. He says the injections take the burden off the patient to take a pill every day.

"I think the key issue is that this once-a-month injectable form is effective, and it really does address that issue of compliance and remaining motivated on a day-to-day basis to take medication, whereas if one's motivated one day a month to get an injection, they don't have to maintain that motivation 29 days of the month," Mr. Garbutt says.

Experts say naltrexone, known as an opioid antagonist, works by dampening pleasure centers in the brain that are switched on by alcohol.

But they say anti-abuse drugs work best if accompanied by therapy.