DUDE! GET A FUCKING ATTORNEY!
Some of these guys know what they are talking about and I would strongly suggest that take their advice.
These cops and this DA are not your friends, and they, SURE AS SHIT, won't become your friends because you cooperate with them. You have no idea what their intentions are, and I wouldn't bank on the idea that this is behind you. You never know, they may want to charge you with something just for the damn hell of it. This DA is not to be trusted anymore than the cops are. In order to get what they want (whatever it is), they will use you and fuck with you in any way they think they can get away with. And, don't think that because you cooperate that everything will turn out alright for you. Don't say anything else and get an attorney. Let that attorney represent you in this matter.
With regard to this situation, give these things some thought:
1) Like you said, how did the cops find out about your prescriptions? That's supposed to be protected by patient-doctor confidentiality.
2) What aren't they telling you about this situation? It sounds to me like they are keeping you in the dark and using you against whoever this is and yourself without you even knowing it.
3) You have no idea that if some point they want to build a case against you. The more cooperate, the more ammunition you give them.
GET A LAWYER!