What I have read about Naltrexone pros and cons:
It is approved by the FDA for alcholism and opiate withdrawal (350 mg) only.
However, unfortunately no phase III clinical trials have been done on Naltrexone being a cure for MS. Clinical trials in general have been small and limited as improving and relieving symptoms of MS as far as pain and mobility. Studies are varied, i.e, Complex Regional Pain Syndrome, fibromyalgia. Forums on the Internet regarding Naltrexone have had patients stating excellent outcomes especially those with PPMS. Certain forums have even stated that lesions decreased on MR scans.
Before starting any new medication, I would strongly recommend asking your Neurologist his opinions and recommendations especially since you have had no flare-ups, exacerbations or progression of your disability.
In our case, my husband has primary progressive MS for the last three years with unremiting pain and neuropathy and progressing ambulatory dysfunction relying heavily on a 4 prong cane. We have decided with our neurologist's approval and with the neurologist following him closely (i.e., LFTs bloodwork as Rx can cause transient and/or elevated liver enzymes) to start LDN (5 mg) daily.
I will keep you informed of the outcomes! My prayers and best wishes on your continued success and great managment of your MS!