Some suspended players, like Giants safety Tyler Sash, have said they had no idea the league considered the drug out of bounds. But Adderall is clearly listed on the NFL's list of banned substances - lumped in the same amphetamine category as greenies and speed on the players' association website.
Adderall may provide a competitive edge but it comes with serious health risks. The stimulant is very addictive and an overdose could result in a drug-induced psychosis or even cardiac arrest. Spanswick noted that abusing Adderall could lead someone to lose touch with reality and have suicidal or delusional thoughts. The FDA classifies Adderall as a Schedule II drug, meaning it has a high potential for abuse. (Drugs like cocaine and morphine are also Schedule II drugs).
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Dangerous drug they're playing with.