treated has 3 distinct synonyms spread across 1 of its 3 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 untreated.
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.
subjected to a physical (or chemical) treatment or action or agent
“the sludge of treated sewage can be used as fertilizer”
given medical care or treatment
“a treated cold is usually gone in 14 days; if left untreated it lasts two weeks”
made hard or flexible or resilient especially by heat treatment
“a sword of tempered steel”
Synonyms, antonyms and the broader and narrower relations for treated come from Princeton University’s WordNet. Full sources and licences.