A high-ranking UK Anti-Doping official has said Maria Sharapova's long-term use of a now-banned substance is "surprising".
Five-time grand slam winner Sharapova announced on Monday she had tested positive for a substance called meldonium at this year's Australian Open.
Meldonium was placed on the banned list by the World Anti-Doping Agency (WADA) at the beginning of the year following "evidence of its use by athletes with the intention of enhancing performance".
Sharapova, who said she had legally taken the medication throughout her career to deal with health issues including an irregular heartbeat, could face a ban of up to four years.
Use of the drug was monitored by WADA last year following suspicions. A study has reportedly shown it was present in the system of 17 per cent of Russians tested and 2.2 per cent of all athletes.
Nick Wojek, head of science and medicine at UK Anti-Doping, said: "It's not a drug used in western Europe and America as a therapeutic drug, so you'd think that's a very high percentage (2.2 per cent).
"I think it would infer, although you couldn't categorically say one way or the other, that there would be some misuse within that number, hence why it's gone onto the banned list.
"If you're using a drug for a non-therapeutic reason then for me that's an unethical use of that particular drug."
Sharapova claimed she was prescribed meldonium, also known as mildronate, by her doctor in 2006 to deal with health issues such as an irregular heartbeat and a history of diabetes in her family.
Wojek said: "From what I've read so far, it's normally used in Eastern Europe to treat stroke patients and patients with heart disease such as angina and heart failure.
"So, outside of that, you're getting into a doctor prescribing something off label, which is possible if it's in the best interests of the patient, but you're getting into areas of using medication for what it hasn't got the licence for."
One of the other major questions arising is why Sharapova was prescribed a medicine that is not licensed in the United States, where she has been based since the age of seven.
The drug company that manufacturers it, meanwhile, has reportedly stated it should normally only be taken for between four and six weeks at a time.
Wojek said: "From the (patient information) leaflet it would suggest they did recommend it's only for a six-week period to help ease the lack of blood flow - depending on what the medical condition was and the dosage being used.
"You'd have to look at the specifics but, generally speaking, if the information leaflet is saying six weeks, (to be prescribed it for 10 years) would be surprising."
The International Tennis Federation (ITF) said Sharapova had been informed of the positive test on March 2 and she will be provisionally suspended from March 12.
Sharapova has waived the right to have her B sample tested and she and her team will instead hope everything she has done so far will limit the sanction she is given.
The next step is for an independent tribunal to be established to hear the case. A decision would be expected around two weeks after the hearing.
Former UK Sport anti-doping chief Michele Verroken says the 28-year-old Russian could receive some leniency if she can prove she needed to take the drug for medical purposes.
Verroken said: "The challenge facing Maria Sharapova and her team is to bring forward the diagnostic evidence that she has a condition that required the prescription of this treatment.
"But if she can actually prove that she may get some leniency from the disciplinary panel."
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