Yellow Xanax Bars R039 is a strong Xanax formulation containing 2mg Alprazolam pills. This oral prescription is one of several medications used to treat anxiety, GAD, and panic disorder. Yellow Xanax Bar has the imprint R039 and three grooves to divide the bar into three equal parts. Because of the similarities in shape and size, this drug is also known as a "School Bus." Furthermore, it is a member of the benzodiazepine class of drugs, which act as (CNS) central nervous system depressants to calm the brain and nerve cells.