Green’s Dictionary of Slang

(drugs) paraldehyde, a polymer of aldehyde (C6H12O3) used both as a narcotic and a legitimate treatment for insomnia.

1969		R.R. Lingeman Drugs from A to Z (1970).
1974		G.V. Higgins Cogan’s Trade (1975) 19: They’s both on paraldehyde [...] he puts some of the paracki in the glass and some water and they sit there and sip it.