Just make the device have an internal clock and dispense a dose every X hours.

Forever? What if you want to change the dosage.

I'm still not seeing a particular need for a crypto key.

So that only the person who's supposed to control it can control it. You don't want someone altering it with their laptop just because they have bluetooth.

Edit:

Somehow I was thinking of implanting something that dispensed drugs. Just dispensing pills would make most of that pointless. Why worry about someone breaking it with a laptop if they can break it with a hammer? I suppose it might work if you somehow build the thing like a safe.