I can't think why anyone would want some of the more common drugs Gran might take. I'm on Lipitor, and it doesn't do a thing except lower your cholesterol. I feel no different when I forget to take it, than when I remember.
But any of the more common oral prescription psychoactives (Valium, Halcion, Ambien, Abilify, Percocet, Darvocet) might attract a teenager. Painkillers are always popular because of their tranquilizing effects.
If you need more detail, there is an online version of the Physicians' Desk Reference or PDR that will give you drugs by class and by name. Warning:it's very technical but should help you decide if a drug is oral (p.o.), injectable (i.m.) or intravenous (i.v.).