pain has 15 distinct synonyms spread across 7 of its 7 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 pleasure.
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 symptom of some physical hurt or disorder
“the patient developed severe pain and distension”
emotional distress; a fundamental feeling that people try to avoid
“the pain of loneliness”
a somatic sensation of acute discomfort
“as the intensity increased the sensation changed from tickle to pain”
a bothersome annoying person
“that kid is a terrible pain”
something or someone that causes trouble; a source of unhappiness
“washing dishes was a nuisance before we got a dish washer”
Synonyms, antonyms and the broader and narrower relations for pain come from Princeton University’s WordNet. Full sources and licences.