dead nettle has 7 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. 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.
foul-smelling perennial Eurasiatic herb with a green creeping rhizome
any of various plants of the genus Lamium having clusters of small usually purplish flowers with two lips
coarse bristly Eurasian plant with white or reddish flowers and foliage resembling that of a nettle; common as a weed in United States
a plants of the genus Pilea having drooping green flower clusters and smooth translucent stems and leaves
Synonyms, antonyms and the broader and narrower relations for dead nettle come from Princeton University’s WordNet. Full sources and licences.