I, personally, take Zanaflex (Tizanidine)...it works for me. I, also, know there are MJ derivatives.
But I do believe folks who say that MJ works for them when certain drugs fail to help them.
Not every drug is effective for every person.
And as far as side effects...heck, we MS patients are shooting ourselves up with interferons on a regular basis...side effects from MJ would not be something that would concern me.
Zanaflex is not a cannabiniod, it's a a2-adrenergic agonist. Did non-interferon therapy ever worked for you (eg: Copaxone)?