Green’s Dictionary of Slang

(drugs) Diconal.

1983		New Society 11 Aug. 203/1: Rit (Ritalin, a stimulant), Dike (Diconal, a painkiller), and many more – you can them all on the streets round here.
2002		(con. 1981) W. Self Dorian 47: Dikes an’ Rits, Dikes an’ Rits, Dikes an’ Rits, five’ll get you three of one, ten’ll get you two of the other.