Dichlorphenamide-13C6is intended for use as an internal standard for the quantification of dichlorphenamide by GC- or LC-MS. Dichlorphenamide is a sulfonamide and an orally bioavailable carbonic anhydrase (CA) inhibitor (Kis = 1.20, 38, 50, and 50 nM for the human CA isoforms CAI, CAII, CAIX, and CAXII, respectively).1It lowers intraocular pressure in rabbits when 50 μl of a 10% solution is applied topically to the eye.2Dichlorphenamide rescues the potassium deficiency and prevents insulin-induced paralysis in a rat model of familial hypokalemic periodic paralysis when administered at a dose of 5.6 mg/kg per day for ten days.3Formulations containing dichlorphenamide have been used in the treatment of glaucoma and primary periodic paralysis.