phase iv clinical trial has 1 distinct synonym spread across 1 of its 1 sense. 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.
sometimes the FDA approves a drug for general use but requires the manufacturer to continue to monitor its effects; during this phase the drug may be tried on slightly different patient populations than those studied in earlier trials
Synonyms, antonyms and the broader and narrower relations for phase iv clinical trial come from Princeton University’s WordNet. Full sources and licences.