Yeah, I prefer the idea of a small implant that would be connected to your veinous system, which you could press down on, and have the drugs delivered into your system, but there would be a limit to how frequently you could administer, and you could adjust it as well. Say the implant contained cocaine, heroin, methamphetamine etc.. Well, I would program it to only allow me to have five doses of dope a day, each one to be administered no less than three hours apart. For the cocaine I would program this "chip" to dose myself four times in one hour, but only allow myself this privilege two times a day.
Amd if I felt like this frequency was too much, or I had to take a break from a certain drug, I would then set it so I couldn't use say, "cocaine" for X ammount of days.