Many Patriots fans were disturbed by the four-game suspension recently imposed on defensive lineman Jermaine Cunningham, reportedly for the use of the common attention-deficit drug Adderall. In fact, the National Football League’s rules against the amphetamine are sensible, but Cunningham’s case highlights the difficulties that the drug poses for the NFL and its players union.

A league source told the Globe that Cunningham tested positive for the drug, which can be used simply as a performance enhancer; some players take it to stay focused in training camp, for instance, or to reduce fatigue in the fourth quarter. For these reasons, the drug is forbidden in the NFL, except when a player with a diagnosed attention-deficit or hyperactivity disorder applies for a therapeutic exemption.