Green’s Dictionary of Slang

(drugs) phyceptone, methadone.

1968		F. Dawtry Social Problems of Drug Abuse in Spears (1986).
1971		Guardian 30 Mar. 11/6: There is thus now a completely new group [of drug addicts in Britain] on methadone, or ‘phy’ (physeptone is the British word for which the World Health Organisation title, methadone, is being substituted).
1973		Times 22 Mar. 6/2: She said to him: ‘Do you want some phy (Physeptone)?’ and made it quite clear that she meant the drug.
1996		P. Howard The Joy (2015) [ebook] I’m going to swap me heroin dependency for a physeptone one, swap a socially-unacceptable addiction for one which the Government will actually pay for. Get me bottle of phy every week for the rest of me life.