Green’s Dictionary of Slang

1. a tablet; modern use usu. one containing a hallucinogenic drug.

1924		Truth (Brisbane) 22 June 12/3: A medical man gave her a prescription which included a quantity of the joyous little half-grain ‘tabs,’ [of morphine] which may be taken internally, or made into a solution in water, and then injected .
1969		I. Hebditch ‘Weekend’ unpub. thesis in Hewitt (2000) 133: I see Harry and get my tabs from him – thirty ‘French’ Blues at sixpence a time.
1972		Nova Apr. 83: An acid tab is about £2.
1984		P. Barker Blow Your House Down 140: Her and her bloody tabs, they’d be the death of her.
1996		P. Howard The Joy (2015) [ebook] Duffo showed me a bag of heroin for the first time. ‘A thousand times better than a tab,’ he said.
1998		(con. 1986) G. Pelecanos Sweet Forever 139: The white dude looked like some mad professor and shit, lit by a hundred tabs of speed.
1998		I. Welsh Filth 100: I think those benny tabs have a high bi-carb content.
1999		Observer 24 Oct. 29: We [...] dropped a tab, while keeping our dilated pupils alert for the fuzz.
2002		J. Cameron Hell on Hoe Street 17: Then she took some tabs out. ‘And these being malaria pills,’ she goes.
2010		K. Richards Life 209: A minute amount of amphetamine [...] and in Robert’s case, heroin tabs.

2. a dose of LSD in non-tablet form; thus tabbing adj., taking LSD.

1968		N. von Hoffman We are the People Our Parents Warned Us Against 41: There are other stories about ‘tabbing parties,’ the long sessions when the vodka-acid mix is put, a drop at a time on each of a thousand pills. By the time the party’s over, everybody, owing to the touching of fingers to lips, is stoned.
1981		D.E. Miller Bk of Jargon 344: tab: A dose of LSD in tablet form, although it may not be a tablet proper but a small cut section of LSD-impregnated paper, sugar cube, or the like.
2006		G. Iles Turning Angel 156: They’d done three tabs of acid in the past twelve hours.
2004		R.G. Barrett Rosa Marie’s Baby (2013) [ebook] Snort a few lines of coke, drop a tab of acid and do a bit of crystal meth.