A hair-loss drug may have cost Mark Nielsen his career after the Kiwi tennis startested postive for a banned substance.

A urine sample taken in Melbourne during Australian Open qualifiers on January 12 found traces of finasteride, banned by the International Tennis Federation (ITF) last year because it can act as a masking agent.

Finasteride is found in hair-loss products, like Propecia, and has been linked to other doping scandals overseas. It is also used to treat anenlarged prostate.

Argentinian tennis player Mariano Hood was banned for a year after testing positive for the same substance at the French Open in May last year.

Nielsen wasn't commenting yesterday, but his Queen's Counsel Kit Toogood said the Kiwi number 1 was not a drugs cheat and was naturally "gutted and devastated" by what had happened.

Nielsen had "inadvertently taken the substance as part of prescribed medication which he had been taking since 2003".

Since the announcement of the positive drug test, Nielsen had withdrawn from the Davis Cup team to play Kazakhstan in April, as well as all other competition.

He would face an International Tennis Federation disciplinary hearing in May at which Toogood believed a one-year ban was the best-case scenario.

"He is devastated because he wanted to play Davis Cup.

"He is now hoping for a fair hearing," Toogood said.

He said a year away from tennis and the lost ranking points would make it difficult for Nielsen to regain his current world standing. Nielsen is 329th in the world and has career earnings of $146,716.

Toogood refused to confirm whether Nielsen had been taking a hair-loss drug, saying only that it was prescription medicine he had been taking for several years.

"I am not prepared to discuss his medical treatment, only to say that Mark did not know it was a banned substance," he said.

His defence to the ITF would be that he had failed to notice finasteride had been added to the list of banned substances last year.

Davis Cup team captain James Greenhalgh told the Herald on Sunday he was shocked when Nielsen phoned him on Friday to tell him of the news.

"He's upset about the whole thing. He feels he's let the team down," said Greenhalgh. He stressed it was Nielsen's decision to step down from the team.

He believed it would not have been in the best interests of the team for him to compete while he was facing a hearing.

Greenhalgh was frantically trying to organise a replacement before the team flew out today for a tournament in South Korea.

"It's a huge setback for us. Obviously it's going to make our job more difficult."

Tennis New Zealand chief executive Don Turner said the drug revelations were a "huge disappointment".

"He (Neilsen) has played for New Zealand for 10 years.

"He's been an outstanding contributor to the Davis Cup programme. He's absolutely committed and plays with real distinction."