You're right, but I would like to clarify something. Buprenorphine and methadone are agonists. So they're opiates. Nealtrexone, on the other hand, is an antagonist, meaning that it blocks the effect of opiates.
Now, I would really like to know why you can have a family doctor in this country, but you can't be prescribed buprenorphine, for example. It was possible in France during the heroin crisis in the 1990s.
Naltrexone is an antagonist, which means it's the opposite of buprenorphine, but it's one of the only drugs available that can be used to treat methamphetamine and opiate addiction in Europe and the U.S. Some people can't or don't want to take buprenorphine, so naltrexone is another very effective option for people who don't want to take an opiate for several years. It's been prescribed in the U.S. for over 20 years.