It's been a while since I ran across this option. Here's a random site about this therapy:
The active ingredient costs <$2, so Vivitrol is 90% price gouging.
Edit: IMHO, Naltrexone is a good prescription drug that actually helps people physiologically. People's pain receptors get overloaded; naltrexone helps to reset them. Naltrexone helps with alcoholism, and apparently with meth amphetamine [1] too.
[1] http://newsroom.ucla.edu/releases/ucla-researchers-identify-...
With that said, Naltrexone doesn't address the fundamental emotional problems that underlie most cases of addiction. So it doesn't do them a whole lot of good to force them to take it, without also helping them find stability, the lack of which probably led them to self-medicate in the first place.