Why the 'exercise pill' isn't likely to eliminate the gym any time soon

Recent headlines teased the idea than an "exercise pill" could replace activity.

— -- Recent headlines touting the benefits of an "exercise pill" have teased the idea that it could be possible to skip the gym and stay fit.

But a new study released this month, which takes a closer look at how the drug GW501516 acts on the body's metabolism, shows the alleged benefits aren't all that new and the potential risks could be significant.

The intention of the research, according to one of the study authors, was not to create a pill that healthy people could take to mimic, or substitute for, exercise.

"It was never our intention to encourage the replacement of exercise with any exercise mimetics," said Dr. Michael Downes, a senior staff scientist at the Salk Institute and the co-author of the paper, in an email to ABC News.

"We believe exercise is one of the best solutions to combat many human disease conditions, and nothing can fully replace exercise for its many health benefits."

Despite the well-known risks, some unscrupulous marketers still promote the experimental drug, which has been commonly referred to as "endurobol" and sold on the black market for years as an exercise supplement. It has been a known performance-enhancing drug that at least one major anti-doping agency has warned athletes not to use.

To test the effects of the drug during the study, the authors gave a group of mice a higher dose of GW501516 and for a longer period -- eight weeks instead of four -- than in previous studies. They used sedentary mice and their results showed some promise. At the end of the eight weeks, the sedentary mice that had GW501516 outperformed the sedentary mice who did not receive it; the medicated mice were able to run for about 50 percent more time before "hitting the wall" -- reaching exhaustion or passing out -- compared to their counterparts.

The same trick worked even when the mice weren't exercising, the researchers found, leading to the idea that taking GW501516 could mimic the effects of regular exercise.

They published the first scientific article in 2001, with promising results. But in 2007, the pharmaceutical company abandoned clinical trials after animal studies showed that even low doses led to rapidly developing cancers in animals.

Research has continued on GW501516, and similar drugs, to study its effects on metabolism and relation to cancers over the past decade, with mixed results. In fact, Downes said, in some trials the drug has even appeared to be protective against cancer. But, he added, "further work is needed to bring clarity to this issue."

Until additional research is done and the results are in, most experts in the field agree that it would be a mistake for the public to experiment with such drugs.

Dr. Max Mehlman, professor of health law and bioethics at Case Western Reserve University School of Law, said further study on humans is unlikely in the near future –- and that anyone taking it now is putting their health at serious risk.

"People are taking a big risk when they take pills that have not been adequately studied in humans," Mehlman told ABC News. "No data on human safety now and all we have are safety risks in animal studies."

Dr. Margarita Abi Zeid Daou is a fourth year psychiatry resident at Vanderbilt University Medical Center and a resident in the ABC News Medical Unit.