Could being on prednisone and cellcept (mycophenolate mofetil) for more than a year cause major health problems?

Yes.: If you have been receiving daily prednisone and Cellcept (mycophenolate mofetil) for a year then you doubtless have a major health problem. Your care is managed by trained, experienced physicians who check you & your lab work on a regular basis making certain that known adverse effects of these potent medications don't occur or are caught early before developing into significant problems. Hopefully, they are working well