Naltrexone
"I have been an IV opiate user for many years. I had an insanely large habit ($750-$1000day). After attending rehab, I was put on the Vivitrol injection. I had been clean for 20 days by the time I go the injection. I did still have cravings, as I was so early into recovery and had been using for so long. However, after about a week of being on the shot, I relapsed on 3.5g of heroin, and felt nothing. It can't be overrode. I spent the next year shooting. And finally started to take the pill form. The pill form will block any opiate for 3-5 days (in my experience). I am now 9 months clean, and very rarely have cravings."