My son takes Vivitrol. He swears by it. He has been on since mid-June, and just had his third shot yesterday.
The risk of Vivitrol is that. because it blocks the effects of opiates, an addict who takes percoset, or uses heroin, will not feel high. However, the risk is that an addict will try to "beat" the shot, using more and more- not feeling the effect- and overdose. The risk of overdose (because of the increased sensitivity) in the immediate period after the shot wears off is also increased. So, it's only recommended for patients who are highly motivated to stay clean, and who are involved in an active program of recovery - meetings, IOP, and/or therapy.
It is very expensive ($1000-$1500) a shot, so is only practical for those with health insurance. It also has some side effects; my son's doctor had him try oral naltrexone for two weeks before the shot to make sure he wouldn't experience any problems.