WordNet records no synonym and no antonym for morning-after pill. 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.
a large dose of estrogen taken orally within 24 to 72 hours after intercourse; prevents implantation of a fertilized ovum and so acts as a contraceptive; commonly used after rape or incest
Synonyms, antonyms and the broader and narrower relations for morning-after pill come from Princeton University’s WordNet. Full sources and licences.