In December 2009, a urine test taken at the United States National Time Trial Championships the previous August, in which Zirbel came second, tested positive for dehydroepiandrosterone (DHEA).[2] Zirbel denied having taken the substance knowingly, but he nevertheless voluntarily accepted a two-year suspension, and immediately announced his retirement. His suspension was reduced in March 2011 to 18 months, which had already been served, in recognition of assistance given to the United States Anti-Doping Agency (USADA). He suggested that, despite earlier comments about retiring, he would seek to resume his racing career. He maintains that he ingested the banned product inadvertently.[3]
