Endorphin rhymes with:
morfin, morfin, alfin, alphin, buffin, chaffin, chafin, chalfin, coffin, cyrofin, daffin, dauphin, delfin, diffin, duffin, dystrophin, elfin, gaffin, giffin, goffin, golphin, griffin, guffin, hanafin, hanifin, hannifin, kniffin, laffin, morfin, murfin, myotrophin, olefin, polyolefin, rofin, ruffin, sarafin, serafin, serfin, superfon, sutfin, sutphin, threadfin, tiffin, woodfin