WordNet records no synonym and no antonym for designer drug. 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 psychoactive drug deliberately synthesized to avoid anti-drug laws; mimics the effects of a banned drug; law was revised in 1986 to ban designer drugs
Synonyms, antonyms and the broader and narrower relations for designer drug come from Princeton University’s WordNet. Full sources and licences.