The banned drug that tennis star Maria Sharapova tested positive for has been used by athletes in recent years, a 2015 study found.

Mildronate, also known as meldonium, was developed and manufactured in Latvia to treat ischemia — a lack of blood flow to an organ — and neurodegenerative disorders. The decades-old drug, which is not approved by the FDA, has long been thought to be used by athletes — especially in Eastern Europe and Russia — to boost endurance and aid in recovery.

A study last year verified those suspicions.

Mildronate was the target of a research project funded partially by the Partnership for Clean Competition (PCC), which is backed by the U.S. Olympic Committee, Major League Baseball, the U.S. Anti-Doping Agency and the NFL. An analysis of 8,300 random urine samples revealed 182 (2.2%) contained mildronate.

Mildronate was also part of the World Anti-Doping Agency (WADA) monitoring program — which analyzes substances misused by athletes that are not part of the banned list — last year. (Those who tested positive are not punished under the WADA monitoring  program.) But in October, WADA announced mildronate would be put on the banned substances list effective Jan. 1.

“From an anti-doping perspective, the 2.2% rate in this study was concerning,” Larry Bowers, chairperson of the PCC Scientific Board, said in October. “This figure represents more than twice the overall rate of laboratory findings for a single drug than any of the substances on the (WADA) prohibited list.”

The study showed that the use of mildronate wasn’t clustered in any particular sport and was “found in a wide range of samples.”

“The PCC is proud to have funded a research project last year that showed the prevalence of the use of mildronate for performance-enhancing effects,” PCC Executive Director Michael Pearlmutter told USA TODAY Sports in an email. “The research study results were shared with the WADA and this substance placed on the 2016 prohibited list. This is the goal of the PCC, to be able to provide a flexible research funding process for scientists where results can better inform the world anti-doping community and protect clean athletes.”

Sharapova told reporters Monday that she takes responsibility for the positive test. She said mildronate had been prescribed by a doctor and she was taking it for a magnesium deficiency.

After the press conference, Sharapova attorney, John Haggerty, said “a positive drug test could result in a ban of up to four years” from the International Tennis Federation. But he added that “mitigating circumstances can lead to the elimination of a ban altogether. … We’re still determining what we are going to request of them. I’ve asked them to have a cooperative process.”​​