A unit dosage form, such as a capsule or the like, for delivering a skeletal muscle relaxant, such as cyclobenzaprine hydrochloride, into the body in an extended or sustained release fashion comprising one or more populations of drug-containing particles (beads, pellets, granules, etc.) is disclosed. At least one bead population exhibits a pre-designed sustained release profile. Such a drug delivery system is designed for once-daily oral administration to maintain an adequate plasma concentration—time profile, thereby providing relief of muscle spasm associated with painful musculoskeletal conditions over a 24 hour period.