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.
These days, doping appears to be less common in cycling, he says. But that's because the sport has changed, not because there's more testing for more drugs.
DAVID GREENE, HOST:
Athletes competing at the Olympics in Rio have been taking a hard line against their peers who've been accused of doping. But competitors can actually still engage in a form of doping that is legal. NPR's Jon Hamilton reports on the practice of taking prescription drugs that may improve performance but haven't been banned yet.
JON HAMILTON, BYLINE: Most people had never heard of a drug called meldonium until tennis champion Maria Sharapova admitted using it in March. That admission is why she won't be competing in Rio. But endurance athletes knew about meldonium. And until the World Anti-Doping Agency banned the drug in January, lots of competitors were taking it.
RONALD EVANS: If it's not banned, athletes will use it.
HAMILTON: That's Ronald Evans of the Salk Institute in San Diego. He's an expert on drugs that affect metabolism. Evans says meldonium is just one of a group of prescription heart drugs that could help a runner or a swimmer last longer.
EVANS: Those are really thought to improve blood flow. You improve blood flow, you improve more oxygen getting to the muscles that you want. And therefore, it's good for performance.
HAMILTON: Meldonium was around for decades before it got banned. And since the ban, it's been found in samples from about 100 athletes. Meanwhile, competitors can still legally use another prescription heart drug called telmisartan. Evans says this drug also improves blood flow in a way that could give an athlete an advantage.
EVANS: It is not prohibited. And if it's not prohibited, that means you almost have to seriously consider it unless you are so much better than your competitors.
HAMILTON: The World Anti-Doping Agency has been monitoring telmisartan since 2015. But Evans says meldonium and telmisartan are just bit players in the doping world. It's not even clear how effective they are. What athletes really want, he says, is the kind of drug he's been working on for years.
EVANS: A drug that promotes the benefits of fitness without actually training.
HAMILTON: Evans isn't trying to help dopers. He's trying to save the lives 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.
EVANS: I get emails from athletes, coaches, the horse racing industry (laughter).
HAMILTON: Doping authorities are also interested in new drugs that can alter metabolism. Olivier de Hon is the manager of scientific affairs for the Anti-Doping Authority in the Netherlands.
OLIVIER DE HON: It is true that over the last few years, yeah, there are more and more inventive medicines which might be performance-enhancing as well.
HAMILTON: De Hon says it can take years to assess a potential doping drug. One reason is that it's hard to find information on the performance-enhancing qualities of a substance. De Hon says this was particularly true of meldonium.
DE HON: Most of the scientific literature that's available about meldonium, it's in Russian. And I cannot read that. So I had some problems in making up my mind whether - what we should do with it.
HAMILTON: And de Hon says you are not going to keep athletes from doping simply by banning lots of drugs. It often comes down to the culture of a particular country or a particular sport. De Hon says, take cycling, for example, a sport where doping has been rampant.
DE HON: Within the world of cycling they realized at one point that this is not the sport anymore that they can sell to the public and can sell to the people who pay for them.
HAMILTON: De Hon says doping appears to be less common in cycling these days because the sport has changed, not because there's more testing. Jon Hamilton, NPR News. Transcript provided by NPR, Copyright NPR.