U.S. player Robert Kendrick has been banned from tennis for 12 months after testing positive for a banned stimulant, the International Tennis Federation announced in London on Friday.

The 31-year-old, who is ranked 105 in the world, tested positive on May 21 at the French Open for the stimulant methylhexanamine, a prohibited substance.

The ITF said that while it accepted the player’s explanation that he accidentally ingested the substance as the result of taking an anti-jet lag pill, he will still have to serve out his ban.

The sanctioning body said that Kendrick denied he was trying to enhance performance by taking the medication.

“It is the player’s strict personal duty to ensure that no prohibited substance enters his body, whether as a prescription medication or otherwise, unless he holds a valid therapeutic use exemption for such substance,” the ITF said in a statement.

Kendrick’s ban ends next May 21. His result and prize money from the French Open will be forfeited. He has only played only once since the positive test, losing in the first round in the Netherlands on grass in June.

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