Psychologically. Instead of constantly feeling okay (the pain is gone, you keep getting a prescribed amount and no more), you are reminded every few hours that with the drug the life is good, and without it the life is miserable. But you cannot alleviate the pain unless you break the rules (the prescription).

Once the prescribed regime is broken, and the psychological restraint is broken, too, there's little left to stop the usage from spiraling up, to an overdose.

because you experience repeated cycles of pain, take pills, then relief, then pain, take pills, then relief.