effect has 14 distinct synonyms spread across 5 of its 8 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.
a phenomenon that follows and is caused by some previous phenomenon
“the magnetic effect was greater when the rod was lengthwise”
an outward appearance
“he made a good impression”
an impression (especially one that is artificial or contrived)
“he just did it for effect”
a symptom caused by an illness or a drug
“the effects of sleep loss”
Synonyms, antonyms and the broader and narrower relations for effect come from Princeton University’s WordNet. Full sources and licences.
act so as to bring into existence
“effect a change”