WordNet records no synonym and no antonym for active placebo. 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 placebo used in experimental tests of a drug that has noticeable side effects
“an active placebo mimics the side effects of the experimental drug”
Synonyms, antonyms and the broader and narrower relations for active placebo come from Princeton University’s WordNet. Full sources and licences.