Green’s Dictionary of Slang

a-head n.

[A n.2 + -head sfx (4)]

1. a regular or excessive amphetamine user.

[US]R.R. Lingeman Drugs from A to Z (1970) 27: A-head [...] A frequent, habitual, often exclusive user of amphetamines.
[US]L. Bangs in Psychotic Reactions (1988) 178: There’s A-heads and there’s speedfreaks.
[US]D.E. Miller Bk of Jargon 339: A-head: A habitual amphetamine user.
R. Burns Suicide Club [Internet] They weren’t totally gone like Ahab the A-Head. He was a thirty year old who looked eighty. He used to be a jazz musician, but he’d taken so much amphetamine that his brain sounded like a non-stop record player that’s skipping through all the tracks and playing a couple of lyrics from each of ten different songs.
[US] ‘Weekly Wire’ Boston Phoenix 4 May [Internet] Ondine was a self-described ‘A-head,’ an amphetamine freak, who was so high during much of the taping that listening to him is like putting your ear up to a radio and scanning the stations with the volume cranked up full blast.
Parenting of Adolescents ‘Drug Sl. Dict.’ [Internet] A-head: frequent amphetamine user.

2. a regular or excessive user of LSD.

[US]E.E. Landy Underground Dict. (1972).
[US]A. Shaw Dict. Pop/Rock.
W. plummer Holy Goof 125: Generally the A-head was brought down from his polychromed nirvana by the speed freak’s wired kinetics and logorrhea.