bitter has 16 distinct synonyms spread across 6 of its 12 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.
harsh or corrosive in tone
“an acerbic tone piercing otherwise flowery prose”
English term for a dry sharp-tasting ale with strong flavor of hops (usually on draft)
the taste experience when quinine or coffee is taken into the mouth
4 of the 12 recorded senses carry no synonym, antonym or related term, so they are not shown above. Every sense, with its definition and examples, is on the full entry for bitter.
Synonyms, antonyms and the broader and narrower relations for bitter come from Princeton University’s WordNet. Full sources and licences.