(Reuters) - Free agent outfielder Marlon Byrd has been handed a 50-game suspension by Major League Baseball after testing positive for an illegal performance-enhancing substance, it was announced Monday.

The 34-year-old Byrd began the season with the Chicago Cubs, who traded him to Boston in April but he was released by the Red Sox on June 12.

Byrd will now be put on baseball's restricted list for the duration of his suspension. He blames the failed test on a past medication he took for an undisclosed surgery.

"I made an inexcusable mistake," Byrd said in a statement released by the players' association. "I absolutely did not use (the medication) for performance enhancement reasons.

"I will serve my suspension, continue to work hard and hope that I am given an opportunity to help a club win later this season."

Byrd batted .270 in 34 games with the Red Sox this year. He has played for five MLB teams over 11 seasons.