proactive has no recorded synonym. 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 retroactive.
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.
descriptive of any event or stimulus or process that has an effect on events or stimuli or processes that occur subsequently
“proactive inhibition”
1 of the 2 recorded senses carries no synonym, antonym or related term, so it is not shown above. Every sense, with its definition and examples, is on the full entry for proactive.
Synonyms, antonyms and the broader and narrower relations for proactive come from Princeton University’s WordNet. Full sources and licences.