Prednisone directly causes atrophy of muscle fibers (myopathy) leading to muscle weakness. This is a very common side effect of chronic (a few months or more) prednisone use but can be either mild or severe. Patients usually notice weakness most in the upper legs. It is not a painful condition directly, but weak muscles and their tendons are more easily strained, which can lead to pain. Muscle weakness that is so severe as to involve the muscles of breathing is rare, but leg weakness can lead to shortness of breath with climbing stairs.