Implants that deliver time-released doses of the anti-addiction medication naltrexone have been touted for preventing drug overdoses, but Australian researchers have found at least five fatal overdoses among implant patients, The Age reported Feb. 5.

Researchers at the University of New South Wales' Drug and Alcohol Research Center said that four men and a woman died from overdoses between 2002 and 2004 despite the implants. The users were suspected of taking large doses of heroin to overcome the blocking effect of the naltrexone.

"The big thing that has been claimed is if you are actively in treatment with naltrexone implants you can't overdose, and the fact of the matter is these people did," said study co-author Louisa Degenhardt.

Another expert said that the patients may have overdosed after the naltrexone wore off. "The problem is when you stop using [naltrexone] you become sensitive to the effects of heroin, so that even much smaller doses of heroin than you used to use could be potentially lethal," said Nick Lintzeris of the addiction-treatment program Turning Point.

The research was published in the Feb. 5, 2007 issue of the Medical Journal of Australia.