WordNet records no synonym and no antonym for cold turkey. It is the only member of each of its synsets, 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 blunt expression of views
“I told him cold turkey”
complete and abrupt withdrawal of all addictive drugs or anything else on which you have become dependent
“he quit smoking cold turkey”
Synonyms, antonyms and the broader and narrower relations for cold turkey come from Princeton University’s WordNet. Full sources and licences.