NEW YORK — Outfielder Marlon Byrd was suspended 50 games by Major League Baseball on Monday after testing positive for a performance-enhancing substance.

Major League Baseball said the 34-year-old Byrd tested positive for Tamoxifen, which can reduce side effects of steroid use and increase testosterone. It is often used to treat breast cancer patients.

"I made an inexcusable mistake," Byrd said in a statement released by the players' association. "Several years ago, I had surgery for a condition that was private and unrelated to baseball. Last winter, I suffered a recurrence of that condition and I was provided with a medication that resulted in my positive test. Although that medication is on the banned list, I absolutely did not use it for performance enhancement reasons."

Byrd is currently a free agent, and will be placed on the restricted list for the duration of his suspension, which began immediately. He started the season with the Cubs and was dealt to the Red Sox on April 21. He was designated for assignment by Boston on June 9 and released four days later.

"I am mortified by my carelessness and I apologize to everyone who loves this game as I do," Byrd said. "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."

In 2009, Byrd admitted using supplements provided by Victor Conte, the founder of the Bay Area Laboratory Co-Operative. BALCO was at the center of a wide-ranging scandal involving performance-enhancing drugs that enveloped several top-level athletes, including Marion Jones, Tim Montgomery and Barry Bonds. Conte pleaded guilty to steroid distribution in July 2005 and served four months in jail.

Byrd said the supplements were all OK to use under MLB rules. He had never before been suspended for failing an PED test.

He hit .210 in 47 games with the two teams, though he hit .270 with a homer and seven RBIs in 34 games with Boston. The Red Sox picked him up when they had a shortage in the outfield after a rash of injuries.

"He played here and he played well," Red Sox manager Bobby Valentine said. "I had no indication or I don't think anyone did."

The 34-year old has played for five teams in 11 major league seasons and is a career .278 hitter.

Byrd finished fourth in the National League Rookie of the Year voting in 2003 with the Phillies and was a National League All-Star with the Cubs in 2010.