Two-time supercross champion James Stewart was suspended from competition after failing a drug test earlier this year, the International Motorcycling Federation, or FIM, said Wednesday.
Stewart and his team, Chino-based Yoshimura Suzuki Racing, confirmed the penalty and said they plan to appeal.
The development came on the eve of the 2015 season for supercross, the stadium version of off-road motorcycle racing, or motocross.
The season opens Jan. 3 at Angel Stadium and ends four months later. Stewart's 16-month suspension, which is retroactive to when Stewart failed the drug test last April, extends until Aug. 11.
"We plan to appeal as I personally feel the punishment far outweighs the situation," Stewart, a Floridian who turns 29 on Sunday, said in a statement. "I'm extremely disappointed to say the least."
The Swiss-based FIM said a urine sample from Stewart obtained after the supercross race in April in Seattle revealed amphetamines, a prohibited substance in the sport.
Stewart's team said the amphetamines came from Stewart's prescribed use of Adderall to treat an unspecified "long-term condition." Adderall is used to treat attention deficit hyperactivity disorder, among other things.
Despite the suspension, Stewart plans to be at the season-opening race in Anaheim to support his team, sponsors and fans, Yoshimura Suzuki Racing said.
Stewart won the supercross championship in 2007 and 2009, and he won the motocross title in 2008.