Mephenesin (INN), also called myanesin,[1][2] is a centrally acting muscle relaxant.

Mephenesin however presents with the major drawbacks of having a short duration of action and a much greater effect on the spinal cord than the brain, resulting in pronounced respiratory depression at clinical doses and therefore a very low therapeutic index.

[3] Mephenesin was the inspiration for the synthesis of a derivative of 1,3-propanediol, meprobamate, by Bernard Ludwig and Frank Berger,[4] the first tranquilizer to see widespread clinical use.

Mephenesin is no longer available in North America but is used in Italy and a few other countries.

[7] Mephenesin was previously used in France as an OTC muscle relaxant called Décontractyl but was taken out of production by Sanofi Aventis and due to a French Health Ministry decree in July 2019.