preventative has 12 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.
remedy that prevents or slows the course of an illness or disease
“the doctor recommended several preventatives”
any obstruction that impedes or is burdensome
an agent or device intended to prevent conception
tending to prevent or hinder
preventing or contributing to the prevention of disease
“preventive medicine”
Synonyms, antonyms and the broader and narrower relations for preventative come from Princeton University’s WordNet. Full sources and licences.