The chairman and chief executive of Maria Sharapova's racket provider has questioned the decision by the World Anti-Doping Agency to add meldonium to its list of banned substances after the Russian star failed a test for the drug earlier this year at the Australian Open.

Sharapova has lost some of her main sponsors after the announcement earlier this week, but Head have stood by the world's highest-earning female athlete and given her the benefit of the doubt.

The company's boss Johan Eliasch believes that WADA should have not fully banned meldonium but imposed a "dosage limitation only".

"In the absence of any evidence of any intent by Maria of enhancing her performance or trying to gain an unfair advantage through the use of [meldonium], we further conclude that this falls into the category of 'honest' mistakes," PA quotes Eliasch as saying.

"Furthermore, we question WADA's decision to add meldonium to its banned substances list in the manner it did; we believe the correct action by WADA would have been to impose a dosage limitation only. In the circumstances we would encourage WADA to release scientific studies which validates their claim that meldonium should be a banned substance.

"The honesty and courage she displayed in announcing and acknowledging her mistake was admirable. Head is proud to stand behind Maria, now and into the future and we intend to extend her contract. Maria may have made a mistake, but she has earned the benefit of the doubt and we are extending it to her. We look forward to many more years of working with Maria."

Men's world number two Andy Murray, who also uses Head rackets, said yesterday that he found it "strange" and "off" that so many athletes use a drug primarily meant for treating heart conditions.