Green’s Dictionary of Slang

1. (drugs) any drug that comes primarily in pill form, barbiturates, amphetamines etc; thus prescription reds, Seconal [the SE prescription that is issued for it].

1980

E. Folb Runnin’ Down Some Lines 168: The commercial capsules were called prescriptions or prescription reds.

2. (drugs) a marijuana cigarette [? ‘just what the doctor ordered’].

2001

ONDCP Street Terms 17: Prescription — Marijuana cigarette.