Flexabenz Plus is a muscle relaxant. It works by blocking nerve impulses (or pain sensations) that are sent to your brain. It is also used together with rest and physical therapy to treat skeletal muscle conditions such as pain or injury.Pharmacodynamics. Flexabenz Plus, a centrally-acting skeletal muscle relaxant, is structurally related to tricyclic antidepressants, thus they share similar properties. It acts on the brain stem, decreasing tonic-somatic motor activities influencing both the α and δ motor systems. It is used as an adjunct in the symptomatic treatment of painful muscle spasms associated with musculoskeletal conditions.