From BR Bullpen

Amphetamines are a class of performance-enhancing drugs which have been used by baseball players for many years. A stimulant, they were referred to as "greenies" in the 1970s and users included Willie Stargell, Dave Parker, Jim Bouton and Bill Madlock. Willie Mays is frequently alleged to have used them as well and they were publicly distributed in some locker rooms. In the 1980s, Major League Baseball began cracking down on amphetamine usage. By the early 21st Century, they had largely fallen from the headlines as weight-gain steroids drew more attention. They hit the news again in 2006 when Jason Grimsley admitted to formerly using them.

Certain types of amphetamines have legitimate medical uses, such as the treatment of attention-deficit disorder. Players who require such treatment are required to request a therapeutic exemption allowing them to take amphetamine-based drugs under the supervision of a physician. However, they sometimes fail to fill in the proper paperwork - or so they claim - and this can lead to suspensions as well n the case of repeat offenses. That was the case with Chris Davis in 2014.