San Francisco Giants reliever Guillermo Mota was suspended for 100 games on Monday, becoming just the third major league player penalized twice for positive drug tests.

The commissioner's office said the 38-year-old right-hander tested positive for Clenbuterol. In November 2006, while with the New York Mets, Mota was suspended for the first 50 games of the next season.

Mota's agent Adam Katz said in a statement that the Clenbuterol was in children's cough medicine.

"Players are responsible for what they put in their bodies. Guillermo understands that," Katz said. "A 100-game suspension for taking a children's cough medicine that contains trace amounts of a prohibited substance, which is what happened here, is severe and unfair and does not reflect the intention of the Joint Drug Prevention and Treatment Program. We will appeal it."

The Major League Baseball Players Association filed a grievance challenging the suspension that will be heard by an arbitrator. Under baseball's drug agreement, grievances for initial positive tests are heard before a suspension is announced but cases involving second or third positives are argued after the penalty is made public.

Outfielder Manny Ramirez and catcher Eliezer Alfonzo are the only previous players to twice test positive. No player has tested positive a third time, which would result in a lifetime ban.

Mota was 0-1 with a 5.06 ERA in nine games for the Giants this year. This is his 14th season in the majors.

"The Giants are disappointed to learn of Guillermo Mota's suspension," the team said in a statement.

Mota has been a setup man and middle reliever throughout his career. He is 39-45 with 10 saves in 726 games while playing with Montreal, the Los Angeles Dodgers, Florida, Cleveland, the New York Mets, Milwaukee and San Francisco.

This is Mota's third season with the Giants. He was with the Mets when he previously suspended and missed the first 50 games of the 2007 season. At the time of that suspension, the identity of a substance causing a positive test is being made public under the new collective bargaining agreement.

Clenbuterol acts as a stimulant, increasing heart rate. In medicine, it is used to treat asthma. Like some steroids, the drug also has anabolic effects. Athletes and body builders use it to build muscle and burn fat.

Ramirez served a 50-game suspension while with the Los Angeles Dodgers in 2009, then retired rather than serve a 100-game ban while with Tampa Bay last year. Because Ramirez sat out nearly all of 2011, his penalty was cut to this first 50 games of this season under an agreement between management and the union that allowed him to end his retirement. He agreed to a minor league contract with Oakland.

Alfonzo was suspended for 50 games in 2008 while with San Francisco and for 100 games last September while with Colorado.