Mammoth Uncensored Word List:
14 words ending with is, 18 letters long

angiostrongyliasis campylobacteriosis coccidioidomycosis glomerulonephritis glomerulosclerosis granulocytopoiesis hypochondrogenesis immunopathogenesis keratoconjunctitis lympohistiocytosis photomorphogenesis pseudofolliculitis pseudotuberculosis zygomaticofacialis