Byrd, who is a free agent, will be placed on the restricted list for the duration of his suspension.
"I made an inexcusable mistake," Byrd said in a statement. "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," he 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."
Cubs players were surprised at the news.
"I'm surprised and feel bad for him because he was a great teammate," Alfonso Soriano said. "I feel sad when you hear names and drug tests, no matter who's the guy, whether he's a bad or good teammate, you feel bad because he's part of the family."
The Cubs traded Byrd to the Red Sox on April 21 for reliever Michael Bowden, and the outfielder was released on June 12. Will it be tough for him to come back?
"Especially after getting released and now getting the [positive] drug test, it'll be a little difficult," Soriano said. "Who knows?"