[Blindedwino]

Yes, I had a friend who was an ex heroin addict, she needed huge quantities of any sort of painkillers to get any effect.

[epmaybe]

It goes back to basically any medication that acts on G Protein Coupled Receptors (of which opioid Receptors fall under). If given the same dose for over 10 days, by day 3 you are seeing a significant loss of analgesic effect.