Sorry you are having such a rough go of it! I am on Avonex and was on Rebif but they are similiar to betaseron. As I understand it, it depends on the person, some have side effects, some don't and there is no rhyme or reason to it. The Avonex people have told me it can vary from shot to shot but usually the side effects lessen over time.
So far, I am not having any lessening of my side effects! I don't know much about amantidine but you could ask the pharmasist if it could contribute to your betaseron side effects.
Take care, and hang in there!
"Life shouldn't be a journey to the grave with the intention of arriving
safely in a pretty and well preserved body, but rather, to skid in
broadside, thoroughly used up, totally worn out, and loudly
shouting...."Wow! What a ride! Thank You Lord!!!"