WOW> I about had a heart attack when I read what your doc wrote. By definition that isn't LOW dose naltrexone.
I don't know where you are and what kind of doctor you are seeing. It might be worth a second opinion. LDN (as others mentioned) must be compounded. It has been used successfully by MS patients (my husband did for years) and now is being used in CRPS though I believe it is still considered trial or experimental so to speak (I'm not using the right words).
Anyway, LDN is very safe (up to 4/4.5 mgs) with little to no side effects which is why many doctors are willing to prescribe it. However you cannot take with any opioid pain relievers.