When tennis star Maria Sharapova admitted in March to having taken the heart drug meldonium, the public got a rare glimpse of a common practice that's often called "legal doping."

It involves taking a legal prescription drug that may improve performance, but hasn't been banned by anti-doping authorities. And lots of athletes competing in the Rio Olympics will be taking advantage of this loophole, doping experts say.

"If it's not banned, athletes will use it," says Ronald Evans, director of the Gene Expression Laboratory at the Salk Institute and an investigator for the Howard Hughes Medical Institute.

Meldonium, which can improve blood flow, was banned by the World Anti-Doping Agency (WADA) in January. So Sharapova's admission in March means she's not eligible to compete in Rio.

But the games are including many other athletes who took meldonium before the ban. The sports they're competing in include tennis, swimming, running, cycling, rugby, rowing and even volleyball.

Since the ban, evidence of meldonium use has been found in tests of more than 100 athletes, according to WADA. But most of these athletes can still compete in the Olympics because levels of the drug were low enough that they might be left over from use that took place before the ban took effect.

Meldonium is just one of a group of prescription heart drugs that could help an endurance athlete gain an edge by improving blood flow, Evans says. "You improve blood flow, you improve oxygen getting to the muscles that you want," he says. "Therefore, it's good for performance."

Meldonium was developed by a drug company in Latvia and has been available since the 1980s. Sharapova reports that she had been taking it for a decade before the ban. And a study of meldonium use at the 2015 European Games in Baku found the drug was used by athletes competing in 15 of 21 sports there.

It's not clear how much advantage those athletes got from meldonium, says Don Catlin, an emeritus professor at UCLA who once ran the university's Olympic Analytical Laboratory. "I'm not convinced at all that it enhances human performance," Catlin says.

But it is clear, he says, that athletes are experimenting with an ever-widening range of prescription drugs in an effort to get an edge.

"All the easy ones have been found," Catlin adds.

One of those drugs is telmisartan, which is marketed as a blood pressure medication.

Like meldonium, telmisartan can improve blood flow. And in 2015, WADA added it to a short list of drugs the agency is monitoring. But telmisartan is not yet prohibited by the group. And that means that, as an athlete, "you almost have to seriously consider using it," Evans says.

Still, meldonium and telmisartan are far from ideal doping agents, Evans says. What athletes really want, he says, is the kind of drug he's been working on for years: "a drug that promotes the benefits of fitness without actually training."

Evans isn't trying to help dopers. He's trying to develop a drug that could save the lives of millions of people with health problems like obesity and diabetes. But he knows that any prescription drug that comes from his work will find its way into competitive sports.

"I get emails from athletes, coaches — the horse racing industry," he says.

That's been true since 2007, when Evans described a compound called AICAR that could dramatically improve the athletic performance of lab rodents. Because of safety issues, it never became an approved drug. Even so, athletes began using AICAR and it was banned by WADA in 2011.

Athletes aren't the only people monitoring drug development. So are the agencies charged with preventing doping.

"Over the last few years, there are more and more [new] medicines [that] might be performance enhancing," says Olivier de Hon, manager of scientific affairs for the Anti-Doping Authority in the Netherlands.

It can take years to assess a potential new doping drug, de Hon says, in part because it can be hard to find information on the performance-enhancing qualities of a substance.

"Most of the scientific literature that's available about meldonium [is] in Russian, and I cannot read that," he says. "So I had some problems in making up my mind what we should do."

Also, simply banning lots of drugs and testing for them is not going to prevent doping, de Hon says. It often comes down to the cultural values of a particular country or a particular sport.

Take cycling for example, a sport where doping has been rampant.

"Within the world of cycling, they realized at one point that this is not the sport anymore that they can sell to the public" or to sponsors, de Hon says.

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