Baclofen (brand names Kemstro, Lioresal, and Gablofen) is a derivative of gamma-aminobutyric acid (GABA). Baclofen is a muscle relaxer and an antispastic agent and is used to treat muscle symptoms caused by multiple sclerosis, including spasm, pain, and stiffness. It is primarily used to treat spasticity and is under investigation for the treatment of alcoholism. Although baclofen is an analog of the putative inhibitory neurotransmitter gamma-aminobutyric acid (GABA), there is no conclusive evidence that actions on GABA systems are involved in the production of its clinical effects. Baclofen is rapidly and extensively absorbed and eliminated. Absorption may be dose-dependent, being reduced with increasing doses. Baclofen is excreted primarily by the kidney in unchanged form and there is relatively large intersubjective variation in absorption and/or elimination. Baclofen is a direct agonist at GABA-B receptors. The precise mechanism of action of baclofen is not fully known. It is capable of inhibiting both monosynaptic and polysynaptic reflexes at the spinal level, possibly by hyperpolarization of afferent terminals, although actions at supraspinal sites may also occur and contribute to its clinical effect.