Green’s Dictionary of Slang

(drugs) ketamine.

1978		L. Kramer Faggots 161: It’s called Super K, it’s from England, it’s a pre-op sedative used for children.
1994		I. Welsh ‘A Smart Cunt’ in Acid House 225: It’s ketamine. Special-Ks like.
1997		N. Barlay Curvy Lovebox 187: Special K, Es . . . Apples.
2001		J. Stahl Plainclothes Naked (2002) 43: Dr. Roos, a bent plastic surgeon he’d busted for selling ketamine two years ago. It turned out Special K was the least of it.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 173/2: special K n. Ketamine.
2005		Indep. 5 Dec. 8: Ketamine is a horse tranquiliser. Clubbers who use it to get high call it Special K.
2009		P. Temple Truth 290: Drug crims, making unthinkable amounts of money from selling ice, GBH, Special K, ecstasy.
2015		T. Pluck ‘Big Snip’ in Life During Wartime (2018) 19: [K]etamine was called Special K, and it was used both recreationally and as a date rape drug.