Is it farther or further?
1771, from Latin relaxantem (nominative relaxans), present participle of relaxare (see relax). As a noun from 1832.
relaxant re·lax·ant (rĭ-lāk'sənt)n. Something, such as a drug or therapeutic treatment, that relaxes or relieves muscular or nervous tension. adj. Tending to relax or to relieve tension.