WordNet records no synonym and no antonym for menthol. 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 crystalline compound that has the cool and minty taste and odor that occurs naturally in peppermint oil; used as a flavoring and in medicine to relieve itching, pain, and nasal congestion
a lotion containing menthol which gives it the smell of mint
Synonyms, antonyms and the broader and narrower relations for menthol come from Princeton University’s WordNet. Full sources and licences.