reactive has 1 distinct synonym spread across 1 of its 2 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. Its one recorded opposite is unreactive.
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.
participating readily in reactions
“sodium is a reactive metal”
reacting to a stimulus
“the skin of old persons is less reactive than that of younger persons”
Synonyms, antonyms and the broader and narrower relations for reactive come from Princeton University’s WordNet. Full sources and licences.