Green’s Dictionary of Slang

1. dexedrine, a form of amphetamine; thus a dexedrine pill.

1961		T.I. Rubin In the Life 51: A lousy little dex or something.
1966		T. Keyes All Night Stand 120: On Dex, for instance, you would hardly know you had any genitals at all.
1971		Harper’s Mag. Sept. 63: Gets drunk once a month on 3.2 beer. Pops a dex or a bennie occasionally, especially during exam week. Tried marijuana at a be-in last summer.
1981		S. King Roadwork in Bachman Books (1995) 457: Of course it was dope. Bad shit, full of dex or something.
1998		P. Doyle (con. late 1950s) Amaze Your Friends (2019) 16: Full of dexes, Remy and reefer.
2001		ONDCP Street Terms 7: Dex — Amphetamine.

2. dextromethorphan (DXM).

1994		alt.drugs (Usenet newsgroup) 10 Feb. 🌐 With dex (dextromethorphan that is) you almost can’t tell that its a chemical making you feel good.
2014		J. Bacharach Bend of World 159: He’s like, oh, an androgynous emissary from another dimension who’s often reported by people returning from a fourth-plateau dex experience.

3. MDMA [methylenedioxymethamphetamine].

2001

ONDCP Street Terms 7: Dex — [...] methylenedioxymethamphetamine (MDMA).