Green’s Dictionary of Slang

psilocybin mushrooms, used as a recreational hallucinogenic drug, also attrib.

1976		Tampa Bay Times (St Petersburg, FL) 1 July 1/1: Paul, the self-proclaimed expert on ‘schrooms’ [sic] describes the schroom high as a mild psychedelic that he finds quite pleasant.
1988		Dly Tar Heel (Chapel Hill, NC) 13 Apr. 3/1: ‘Shrooms haven’t gotten the bad publicity that other drugs have gotten’ .
1994		L.A. Times 24 Feb. 155/2: ‘I started doing drugs again [...] Mostly bud (marijuana) and shrooms (psychedelic mushrooms).
2000		N. Griffiths Grits 168: Av got a few dried ’shrooms on me, Malcolm sehs.
2004		Yungun untitled track on Louis Slipperz £10 Bag Volume II [album] 🎵 You shouldn’t write rhymes just after you take shrooms.
2008		Simon & Burns ‘Get Some’ Generation Kill ep. 1 [TV script] Man. Wish I had some fuckin’ shrooms. This [i.e. combat] is the fuckin’ shit.
2020		C. Hiaasen Squeeze Me 245: ‘Are you lookin’ for ’shrooms?’.