WordNet records no synonym and no antonym for vasopressor. It is the only member of its synset, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
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.
any agent that produces vasoconstriction and a rise in blood pressure (usually understood as increased arterial pressure)
Synonyms, antonyms and the broader and narrower relations for vasopressor come from Princeton University’s WordNet. Full sources and licences.