painful has 9 distinct synonyms spread across 3 of its 4 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 painless.
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.
causing physical or psychological pain
“worked with painful slowness”
exceptionally bad or displeasing
“atrocious taste”
causing physical discomfort
“bites of black flies are more than irritating; they can be very painful”
Synonyms, antonyms and the broader and narrower relations for painful come from Princeton University’s WordNet. Full sources and licences.