Our MS board here has several people who use "low dose Naltrexone--LDN). There is a sticky thread there:

http://neurotalk.psychcentral.com/thread71392.html

There are several other threads there too, over the past 4 yrs.

You do not need B12 injectable. You can accomplish your goal to raise B12 levels orally. Bcomplex works well orally as well.
If you think you have the DNA mutation (10% of people) who do not methylate B12 or folic acid, there is RX Metanx oral.

People with malabsorption issues, primarily from gluten intolerance/damage, have problems with all nutrients until they go gluten free and heal up.

Your need for Bcomplex and B12 can be tested. Thiamine and B6 have serum tests, and if very low that would point to needing the complex. Same with B12-- anything below 400 needs supplementation.

What exactly is the prescription for...can you read it and put it up here? LDN has to be compounded at special locations.