I don't take Rebif but I do take Avonex. In addition to the differences in side effects related to injection method and dosage, many of the side effects are the same since these are both IFN-B1A.
With Avonex, I've experienced many muscle aches the day after my injection. These range from a general achyness all over to having it be narrowed down to an arm or a leg muscle. These usually go away within an additional 24 hours for me. Lots of sleep helps. Sometimes taking ibuprofen for 12 hours prior, and 24 hours after my injection has helped in the past. Now, 4.5 years later, I can sometimes get by with just a single 600 mg dose of ibuprofen with my injection.
I've also experienced a general overall body muscle ache the day after my injection where it felt like ice pellets hitting my body when I shower. Fortunately, this hasn't happened for some time.
Anyways, hang in there. Your side effects are likely to improve as your body adjusts to the medication. Mine have though it did take considerably longer than the time cited in Biogen's literature.
If you feel your side effects are considerably worse than they should be as indicated in Rebif's literature, then discuss this with your doctor or your doctor's nurse. As an example, my first experience with Avonex produced one of the worst fevers I can recall having. Biogen suggests taking Tylenol before and at the time of the injection. This simply didn't cut it with me and I called my doctor and his nurse suggested that I try ibuprofen instead. This has worked for me whereas Tylenol did not. Your doctor may have a suggestion that works for you.
Good luck and best wishes, NHE