BOSTON -- Outfielder Marlon Byrd, who has not caught on with another team since being designated for assignment by the Red Sox on June 9 and ultimately becoming a free agent, has been suspended 50 games by Major League Baseball after testing positive for Tamoxifen, a performance enhancing substance banned by MLB.

Tamoxifen -- its brand name in the United States is Nolvadex -- is a medicine that blocks the effects of the estrogen hormone in the body. It is used to treat breast cancer in women or men.

Asked if Byrd had failed any drug tests while with Boston, Red Sox general manager Ben Cherington said, “Not to my knowledge.’’

Cherington was asked if he had any comments on Marlon Byrd.

"No," Cherington said. "He played here and he played well. I had no indication. I don't think anybody here did."

Was he tested here or before?

"I do not know that. I can tell you I was just in the training room and they had no idea."

Byrd issued a statement through the Major League Baseball Players’ Association.

"I made an inexcusable mistake,’’ the statement said. “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.

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

Here is how Tamoxifen (Nolvadex) is used by body-builders, according to the website elitefitness.com:

“During a steroid cycle, Nolvadex is used by bodybuilders who are sensitive to estrogen buildup. Estrogen buildup can lead to many concerns, foremost of these is the occurrence of gynecomastia (growth of breast tissues in men) and subcutaneous fat and water retention. Nolvadex is effective in averting the dreaded effect when coming off a steroid cycle -- the post-cycle crash. This is basically losing what you have gained when you’re on a cycle. Bodybuilders, in general, use Nolvadex to block this from taking place.’’

Byrd has been an unapologetic client of Victor Conte, the chief of the notorious BALCO (Bay Area Laboratory Cooperative) who was linked to baseball slugger Barry Bonds and Olympic star Marion Jones among other elite athletes. Jones was stripped of the five medals she won in the 2000 Sydney Olympics and was sentenced to six months in federal prison on charges she lied about using steroids. Conte also did time in 2005 after pleading guilty to charges of conspiracy and money laundering.

Byrd’s continued association with Conte was first reported by Steve Henson of Yahoo!Sports in 2009, a relationship that dismayed MLB.

"We’ve talked to him, and he knows how we feel," commissioner Bud Selig said in 2010. "It’s not a situation that makes me very happy."

After coming to the Red Sox, Byrd acknowledged he still trained with Conte. Asked if he had considered making a change, he responded, “Why would I?’’

But he had suggested in the past that his continued association with Conte could cause him problems.

"I'm always going to watch what I take,’’ he told Bruce Levine of ESPN Chicago in March 2011, while playing for the Cubs. “I'm not going to say I have a bull's-eye on my back, but I think a lot of people are waiting for me to get my first positive test and miss 50 games. They'd like that just so they can say, 'We told you so.' I know that won't happen. I know I'm clean. I know the supplements I take are clean. I'm going to make sure of that."

Byrd came to the Red Sox from the Cubs on April 21 in exchange for pitcher Michael Bowden and a player to be named later (minor-league pitcher Hunter Cervenka) a week after Jacoby Ellsbury went on the disabled list with a partially dislocated shoulder. The 34-year-old outfielder played in 34 games for the Red Sox and posted a .270/.286/.320/.606 batting line before being designated, having been displaced as Boston’s primary center-field option by Scott Podsednik, who came from the Phillies in a minor-league deal.

“He played here and he played well,’’ Red Sox manager Bobby Valentine said. “I had no indication. I don't think anybody here did.’’

Asked whether he had been tested while a member of the Red Sox, Valentine said: “I do not know that. I can tell you I was just in the training room and they had no idea.’’

Byrd, who remains a free agent, will be placed on baseball’s restricted list for the duration of his suspension, which will begin immediately and will be in effect through Aug. 20, 2012.