Green’s Dictionary of Slang

co-pilot n.

[it helps you ‘fly’; also poss. the fact that truck drivers use it to stay awake]

1. (drugs) an amphetamine; usu. in pl.

[US]R.R. Lingeman Drugs from A to Z (1970) 70: copilots dexedrine tablets.
[US]New Yorker 7 May III:1: ‘Go Ask Alice,’ records the entry of 15-year-old Alice into the world of drugs [...] of taking ‘Dexie,’ marijuana, ‘copilots,’ LSD, heroin, everything.
[US]D.E. Miller Bk of Jargon 337: copilots: Dexedrine, an amphetamine.
[US]ONDCP Street Terms 6: Co-pilot — Amphetamine.

2. (drugs) two or more people taking LSD together; more usu. the second person is not taking LSD and is there to look after people.

[US]R.R. Lingeman Drugs from A to Z (1970) 70: copilot One who sits with someone who has taken lsd and cares for him if necessary.
[US]E.E. Landy Underground Dict. (1972).
[US] AS LVII:4 289: A guide does not use drugs during parties but serves as a controller, known variously as groundman, babysitter, tour guide, co-pilot, ground control, guru, travel agent, or just sitter.