Green’s Dictionary of Slang

benz n.

1. (drugs) amphetamine.

1948		J.E. Webb Four Steps to the Wall 24: ‘What do you want?’ ‘Four dozen benzies,’ said Jason [...] ‘They went over big.’.
1959		F. Norman Stand on Me 39: Now from taking all this benz and scrumpy and stuff, I wasn’t all that bright on remembering.
1968		J. Barlow Burden of Proof 19: Many are on stimulants, benz, dex, all that stuff.
1971		E.E. Landy Underground Dict. (1972).
1981		D.E. Miller Bk of Jargon 336: bennies, benz, benzies: Benzedrine, an amphetamine.
2001		ONDCP Street Terms 3: Bens — Amphetamine; Benz — Amphetamine.

2. MDMA.

1999		Guardian Guide 5–12 June 30: What else could explain the re-emergence of car logos on ecstasy tablets? Once it was Mercedes Benz (‘Benz Your Mind’).
2001		ONDCP Street Terms 3: Bens — [...] methylenedioxymethamphetamine (MDMA); Benz — methylenedioxymethamphetamine (MDMA).

In derivatives

benzed (adj.)

under the influence of amphetamine.

1958

F. Norman in Vogue Oct. in Norman’s London (1969) 34: Scouting parties [...] arrived back some days later benzed up to the earholes.