prophylactic has 9 distinct synonyms spread across 5 of its 5 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
capable of preventing conception or impregnation
“contraceptive devices and medications”
warding off
“the swastika...a very ancient prophylactic symbol occurring among all peoples”
preventing or contributing to the prevention of disease
“preventive medicine”
remedy that prevents or slows the course of an illness or disease
“the doctor recommended several preventatives”
contraceptive device consisting of a sheath of thin rubber or latex that is worn over the penis during intercourse
Synonyms, antonyms and the broader and narrower relations for prophylactic come from Princeton University’s WordNet. Full sources and licences.