Sesil Karatantcheva has failed in her appeal to the Court of Arbitration for Sport to overturn a two-year ban after testing positive for nandrolone.
She claimed that the positive test at the 2005 French Open was down to food supplements she took when pregnant.
But the Court said nandrolone was found in concentrations which were inconsistent with the normal levels for that stage of pregnancy.
The International Tennis Federation imposed the ban on 11 January.
The 16-year-old Bulgarian beat Venus Williams on her way to the quarter-finals of the French Open in May last year, before losing to Elena Likhovtseva.
It was after that defeat that she produced the positive test, and then failed an out-of-competition test in Tokyo just over a month later.
The court said the standard two-year ban would apply even though Karatantcheva was only 15 when she produced the positive test.
The court said anti-doping rules must apply "in equal fashion to all participants in competitions they govern, irrespective of the participant's age."