for me personally, opiates go best with slow heavy metal, like doom, sludge, etc. Think Earth, Disappearer, Electric Wizard, Generation of Vipers, Lento, Grasshopper Lies Heavy, Om, Petitpoil, Ufomammut. I also often listen to very heavy psychadelic music when I'm high – Bardo Pond, Comets on Fire, Psychic Ills, Subarachnoid Space, Wooden Shjips.
And when I’m nodding out, ambient music can't be beat.