For an addict to these sorts of painkillers, the addiction to painkillers in large part CAUSES THE PAIN.
Dr. Drew Pinsky, who is a specialist in this area (was an excellent article on Rush and this addiction a few days ago by him posted here) has mentioned that once one is addicted to painkillers, the brain will actually keep producing the pain that the painkillers were prescribed for in order to cause the person to keep taking the drug.....the pain actually ends up lasting longer than it would have if no opiate painkillers were given in the first place.
When he's treated painkiller addicts and got them both detoxed and into long-term rehab he's had patients have their "severe back pain" mysteriously disappear over and over again.
Another thing he's noted is that for whatever reason a lot of "chronic back pain" patients have underlying psychological problems. This isn't saying that EVERYONE with back pain is a mental patient and the pain isn't real, it's just there's an interesting correlation.
But the mind is an incredibly powerful thing. Opiates are SO addicting that the mind will actually create pain to insure that the supply of opiates will continue into the body.
Dr. Pinsky also said that when an addicted person goes off these drugs, it can cause that person feel extremely sad, even grief-stricken for a year.