Calf muscle spasm is a painful muscle spasm, which in some cases requires the use of muscle relaxants. However, the name muscle relaxant is somewhat misleading because this class of drugs does not act directly on muscles. Instead, most muscle relaxants act on the central nervous system, including the brain and spinal cord. Therefore, the most common side effect of muscle relaxants is drowsiness or sedation. Here are some notes when using leg muscle relaxants.