In higher doses, Haggerty said, meldonium can serve as a performance-enhancer. Because Sharapova was taking a prescribed dose for health purposes, he said he would probably request a minimal penalty from the I.T.F.

Ben Nichols, a WADA spokesman, said in an email Monday that meldonium had been moved from the monitored list to the banned list “because of evidence of its use by athletes with the intention of enhancing performance.”

One factor in its change in status was a 2015 study, funded in part by the Partnership for Clean Competition, that analyzed 8,300 urine samples collected at doping control sessions and found that 182 (2.2 percent) contained the substance.

But Haggerty said enhancing performance was never Sharapova’s intent.

“Maria was completely unaware there was any performance-enhancing capabilities to it whatsoever,” Haggerty said. “All the things, the medical conditions she was being treated for with this drug, had nothing to do with enhancing her performance. It had to do with her getting her health to a normal baseline.”

For a first offense, players can be barred for up to four years for intentionally ingesting a performance-enhancing substance. The maximum penalty for unintentionally using a prohibited drug is two years, Haggerty said, but he is holding out hope that no sanctions will be imposed.

“She acknowledged she took the drug called mildronate and that under a different name, meldonium, it is on the banned list,” Haggerty said. “And that is why she’s acknowledged that she’s failed the drug test, and now we are just going through the I.T.F. process to discuss with them why we believe that either no, or a very limited, sanction is required based upon all the facts surrounding why she was taking it, for how long she’s been taking it and the medical issues she was taking it for.”

Tennis’s antidoping program has sometimes been criticized for its failure to catch its stars. But the prominent men’s players Marin Cilic and Viktor Troicki were penalized in 2013, with both having their suspensions reduced on appeal. In the past two years, the program has significantly increased the number of blood tests and out-of-competition tests, although Sharapova’s positive test came in competition.

“She is a megastar,” Ings said. “So it ends the conspiracy theory that tennis has no stomach for big cases.”