Larry Bowers, the chief science officer for the United States Anti-Doping Agency, gave a presentation on Oct. 4, 2014, at the organization’s Symposium on Anti-Doping Science.

Dr. Bowers stood before a small, international group of researchers at the Phoenix Marriott Tempe in the Arizona hills and spoke about a novel drug that was unfamiliar to most in the room: meldonium, which had been developed in Latvia in the 1970s as a heart medication. The concern was that elite athletes were abusing it to enhance their performance through its ability to energize the cell’s powerhouses, the mitochondria.

Dr. Bowers’s talk foreshadowed the latest doping crisis in international sports. Less than two years later, meldonium has had an unforeseen side effect: chaos in global sports, affecting athletes in sports as varied as tennis, speedskating, wrestling and track and field. In the first 10 weeks of 2016, at least 99 athletes have tested positive for the drug, according to the World Anti-Doping Agency, demonstrating widespread use of a substance believed to enhance endurance and aid recovery by improving blood flow throughout the body.

“Ninety-nine is an extraordinary number to get in that short a period,” said Richard Ings, the former chief executive of the Australian Sports Anti-Doping Authority.

On several fronts, the last six months have been a period like no other in the long-running antidoping tussle. Russia was barred from international track and field because of systemic doping, and allegations of widespread corruption have been levied against the top of track and field’s global governing body.

Now, along comes meldonium, a once-obscure drug — not approved for sale in the United States or the European Union but sold over the counter in Russia and some Eastern European countries — whose name suddenly rolls off the tongues of sports fans as easily as those of established performance-enhancing drugs like EPO and human growth hormone.

A positive test from the tennis star Maria Sharapova, one of the world’s most prominent athletes, has driven much of the global interest, but the number of positive tests — expected to soar past 100 this week, if it has not done so already — has experts grasping for context.

“Never seen anything like it,” Max Cobb, the chief executive of U.S. Biathlon, said in a telephone interview. “It’s definitely shocking and disturbing for me personally because I think it’s probably not the only pharmaceutical being used to enhance performance.”

Experts and officials struggled to suggest a precedent, although David Howman, WADA’s director general, referred to 2010, when the stimulant methylhexaneamine was added to the banned list, resulting in a flurry of positive tests. Accredited WADA laboratories would report 123 that year in total. While that number may have been unimposing compared with the 337 positive tests for the anabolic steroid stanozolol in 2010, it did make an immediate impression, partly because of methylhexaneamine’s presence in many nutritional supplements.

“I don’t like to express views on numbers, but I think what you find is that when the substance has been new on the list, there is a time for people to sort it out,” Mr. Howman said. “Methylhexaneamine was a similar situation. There were a number of cases in the first three or four months.”

A Standard Process

Though the banning of meldonium has quickly generated results, its administrative pathway to the WADA list was not an accelerated process.

“It was done in a very scientific way with all the information from papers and so forth received and considered,” Mr. Howman said. “It followed the normal process.”

That process, according to Mr. Howman, began in earnest in early 2014. It also apparently got a push from a tip received by Usada in March 2014 from a person whose identity has remained confidential, claiming that athletes from Eastern Europe were using meldonium to enhance their performance. That tip was reported first by USA Today and was confirmed on Saturday by an American antidoping official who requested anonymity.

The tip led to further research on meldonium, also known as Mildronate, by Usada. Eventually, it led to a research project funded in part by the Partnership for Clean Competition, an organization founded in 2008 by the United States Olympic Committee, Major League Baseball, the N.F.L. and Usada to support antidoping research.

On Sept. 29, 2014, shortly before Dr. Bowers’s presentation in Arizona, meldonium was added to WADA’s 2015 monitoring program, putting it on a list of substances earmarked for extra scrutiny that were being considered for inclusion on the banned list. Michael Pearlmutter, the executive director of the Partnership for Clean Competition, said Dr. Bowers and Mario Thevis then met in Vienna in November 2014.

Dr. Thevis is a German biochemist who works at a laboratory in Cologne and is one of 13 experts who make up WADA’s prohibited-list group, an influential international panel that largely determines which substances are considered for the monitoring list and then for prohibition.

That group is widely known as the list committee.

“They meet every January to look at what might be on the horizon,” Mr. Howman said. “And they use the results of research projects and the results of what’s come in from the labs from the previous year and all these sorts of things to start talking about whether there should be any alterations or amendments to the list.”

Mr. Pearlmutter wrote in an email that on Dec. 1, 2014, Dr. Thevis “submitted an application for funding to the P.C.C. that referenced ‘a growing number of indicators for Mildronate’s misuse in sport has been recognized.’ ”

Mr. Pearlmutter said the partnership approved a grant proposal on Dec. 4, 2014 and the resulting study was completed in early 2015.

The study analyzed 8,300 random urine samples collected at doping control sessions and found that 182, or 2.2 percent, contained meldonium, which athletes were still permitted to use at that stage.

“It was published in late 2015 but was turned over to WADA earlier in the year,” Mr. Pearlmutter said of the study. “We believe that it had an impact on the decision by WADA to move Mildronate from the monitoring list to the prohibited list.”

Other studies have since underscored more dramatically the drug’s widespread use in certain communities. One such study, at last year’s European Games, suggested that nearly 500 of the 6,000 athletes competing were taking the drug. That study was also forwarded to WADA and its list committee.

Mr. Howman said the list committee meets every April to create a draft list, which is followed by a call for wider input from the stakeholders in the antidoping community.

“They ask for submissions or suggestions about what else might be considered,” said Mr. Howman, who said there can be several hundred submissions in a year.

They are submitted electronically through an online portal. The list committee then meets again before determining a final version of the revised list, which is then submitted to WADA’s health, medical and research committee.

“That’s a group of doctors and scientists as well,” Mr. Howman said. “And they then make the determination of whether to accept the recommendation or change it and then report to the WADA executive committee, which makes the final decision so that it is transmitted by the 30th of September. We have to do that to give people three months notice of any changes before it comes into being on the 1st of January.”

Informing Athletes

That three-month window is intended to allow time for word to spread, for athletes with a legitimate medical need for the substance to seek an exemption, and for athletes to clear their systems of newly banned substances.

The high number of positive tests for meldonium has been a reminder of athletes’ continual search for a pharmaceutical edge and has raised questions about the ethics of using a prescription drug for performance enhancement even when it is legal. It also has some wondering whether elite athletes in all parts of the world are being educated sufficiently about imminent changes to the banned list, or if they are simply not using their ubiquitous smartphones smartly enough.

“This generation of athletes competing is so digitally connected, and every athlete in the world is on some noticing program,” Mr. Cobb, the U.S. Biathlon official, said. “It’s very difficult to imagine the information not being properly transmitted to them.”

Ms. Sharapova, who claims to have taken meldonium since 2006 for a variety of medical conditions, has said she takes full responsibility for not clicking on a link to the 2016 banned list that was sent to her by email in December. Her extensive support team apparently also did not notice the rule change or alert her.

“There are also probably a lot of athletes who are pretty negligent and who haven’t read or who have missed the instructions about the changing regulations,” said Tom Bassindale, a senior lecturer in forensic and analytical science at Sheffield Hallam University in Britain. “Word could not have filtered down. That could honestly be an issue, but certainly for the top elite, they would have had that briefing.”

Asked about the possibility that athletes were not getting the message, Craig Reedie, WADA’s president, said in an email, “We take great care to inform our stakeholders of any amendments to the prohibited list.”

Though WADA posts changes to its code on its website, it does not inform athletes directly of changes, relying instead on its partners: national antidoping agencies and international sports federations. The question is whether those bodies have all done a thorough a job of spreading WADA’s word.

Turmoil in Russia’s sports and antidoping community may have created communication problems, but Mr. Howman disputed that there was a link.

“That’s a major reach, because this list went out in September 2015,” he said. He also noted that it was not until November that a report by WADA’s independent commission revealed damning findings on Russian sport.

But Anna Antselovich, the head of Russia’s antidoping agency, told the state-run news agency Tass on Thursday that the sanctions against the agency in November had damaged the information chain on meldonium.

“We had no possibility for a certain period of time to hold educational seminar with athletes, coaches and the personnel of national teams,” she told Tass.

Russian athletes in some sports apparently received notification more expeditiously than others. Sergey Shubenkov, the reigning world champion in the 110-meter hurdles, said he received word through official channels. But Elizaveta Tuktamysheva, the 2015 world champion in women’s figure skating, told a Russian website that she had been taking meldonium before the ban but stopped, only because she found out about the change “from my friends who are skaters.”

Several of the most prominent athletes to test positive for meldonium who are publicly known are Russian, including Sharapova, who has been based in the United States since age 7. But it is not uniquely a Russian issue; athletes from several other countries, including Sweden and Georgia, have also tested positive.

There is also the matter of whether the banning of a drug that has long been legal for use in some parts of the world might require an exceptional level of communication from WADA and its stakeholders.

What seems evident is that with the high volume of meldonium cases now pending, there could be legal challenges as to whether the drug should have been banned by WADA in the first place. But Mr. Cobb and others have not accepted the argument that taking the drug for an extended period before its ban is a strong defense.

“I don’t have a lot of sympathy for people using it who didn’t check the list,” Mr. Cobb said. “As far as I’m concerned, they had found a way to use a performance-enhancing drug that was not being tested for, and that’s a moral line I wouldn’t cross. If you are going to go that route, you better damn well be sure you read the memo.”​