MOSCOW — The drug at the center of Maria Sharapova’s doping case, regularly given to Soviet troops in the 1980s to boost their stamina while fighting in Afghanistan, is normally prescribed for medical use for periods of 4 to 6 weeks.

Sharapova faces possible sanctions after testing positive for meldonium, also known as mildronate, a drug the Russian tennis star said she started using 10 years ago for various medical issues. Sharapova did not indicate in her announcement Monday how often she has taken it.

Her lawyer, John J. Haggerty, declined to go into specifics Tuesday but said during a conference call he wanted “to disabuse the concept that Maria took mildronate every day for 10 years because that’s simply not the case.”

Grindeks, the Latvian company that manufactures meldonium, said in an emailed statement to The Associated Press: “Depending on the patient’s health condition, treatment course of meldonium preparations may vary from four to six weeks. Treatment course can be repeated twice or thrice a year. Only physicians can follow and evaluate patient’s health condition and state whether the patient should use meldonium for a longer period of time.”

Haggerty said “Maria at all times took the mildronate in accordance with the recommendations of her doctor.” He repeatedly declined to go into specifics about Sharapova’s treatment, claiming he had to adhere to the “strict confidentiality” of the International Tennis Federation’s process. An ITF spokesman told The Associated Press, however, that the organization places no restrictions on what players and their representatives can say.

Sharapova, a five-time Grand Slam champion, said Monday she failed a doping test at the Australian Open in January for meldonium, which became a banned substance under the World Anti-Doping Agency code this year.

Meldonium is a heart medicine that improves blood flow and is little-known in the U.S., but it was once common in the Soviet military.

The drug’s inventor, Ivars Kalvins, told Latvian newspaper Diena in a 2009 interview that meldonium was given to soldiers during the 1980s, when Soviet forces were fighting in Afghanistan.

“High altitudes. Oxygen deprivation. If they have to run 20 kilometers with all the gear, at the end they would get ischemia (a blood circulation condition),” Kalvins was quoted as saying.

“They were all given meldonium. They themselves were not aware they were using it. No one was being asked (if they agree to it) back then.”

Kalvins said meldonium was “not doping,” adding that it “allows you to withstand more physical pressure, but the body still spends its spare reserves.”

Sharapova said Monday she had taken meldonium for a decade following various health problems including regular sicknesses, early signs of diabetes and “irregular” results from echocardiography exams.

Meldonium was banned because it aids oxygen uptake and endurance, and several athletes in various sports have already been caught using it since it was banned Jan. 1.