All that tells is that there is a layer a indirection. The friends, family, and dealers to some degree get pills from people that got them from doctors. Some may come from overseas, theft, etc., but if 25% are hooked directly from having been prescribed them, I would guess a big portion of the 75% is people within a few hops away.

Often what happens is that the 25% that got hooked from being prescribed start buying additional pills. Eventually they can't afford that and they switch to cheaper heroin, often funded by selling their prescription pills.