tonic has 15 distinct synonyms spread across 7 of its 9 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 atonic.
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.
employing variations in pitch to distinguish meanings of otherwise similar words
“Chinese is a tonal language”
imparting vitality and energy
“the bracing mountain air”
lime- or lemon-flavored carbonated water containing quinine
a sweet drink containing carbonated water and flavoring
“in New England they call sodas tonics”
2 of the 9 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 tonic.
Synonyms, antonyms and the broader and narrower relations for tonic come from Princeton University’s WordNet. Full sources and licences.