Antonyms are words that have opposite meanings to another word. For the word "counteractants," which refers to a substance that counteracts the effects of another substance, some possible antonyms might include enhancers, enablers, promoters, or boosters. These words all suggest something that enhances the effects of another substance, rather than counteracting it. Another possible antonym might be facilitators, which suggests that the substance in question helps to facilitate or enable a particular process, rather than working against it. Ultimately, the choice of antonym will depend on the context in which the word "counteractant" is being used and the particular shades of meaning that the speaker or writer wishes to convey.