Mammoth Uncensored Word List:
22 words ending with is, 16 letters long

abdominocentesis antiangiogenesis antituberculosis arteriosclerosis cataractogenesis chemoprophylaxis electrocatalysis electrodiagnosis erythroblastosis gastroduodenitis haemochromatosis hypervitaminosis megasporogenesis microsporidiosis neanderthalensis olecranarthritis phlebothrombosis rhinosporidiosis sphingolipidosis spondylarthritis strongyloidiasis thrombophlebitis