Low Dose Naltrexone (4.5mg/day or less) seems to have a positive effect on some people with some variations of MS. Its unlikely that 4.mg would block the effectiveness of pain meds.
Scientists don't really understand why LDN works with MS...the theory is that it triggers the brain to produce endorphins.
The 4.5mg max came about from that study in California. The findings were worded to the effect that doses higher than that did not seem to increase the positive effects,
Personally, since I'm a pretty big boy, I take 4.5mg twice a day now. Its been about a month since I increases the dosage. I can report slight improvement in spasticity, but the improvement does not last more than an hour or two.