Green’s Dictionary of Slang

(drugs) pethidine.

1982		G. Newbold Big Huey 15: It was difficult, given the simplicity of the operation, not to slip into the habit of selling a bit of peth., morph., opium tincture and so on.
2001		ONDCP Street Terms 17: Peth — Depressant.
2005		P. Temple Broken Shore (2007) [ebook] For a long time [...] he had been unable to cope without resorting to pethidine. Getting off the peth, the lovely peth, that was the hardest thing he had ever done.