Is it farther or further?
late 14c., from Old French purgatif (14c.) and directly from Late Latin purgativus, from purgat-, past participle stem of Latin purgare (see purge (v.)). The noun is attested from early 15c. (Old English medical texts have clænsungdrenc).
purgative pur·ga·tive (pûr'gə-tĭv)
An agent used for purging the bowels. adj.
Tending to cause evacuation of the bowels.