sulphamethoxazolePronunciation: /ˌsʌlfəmɪˈtɒksəzəʊl/ (US sulfamethoxazole)Definition of sulphamethoxazole noun [mass noun] Medicine a sulphonamide antibiotic used to treat respiratory and urinary tract infections, and as a component of the preparation co-trimoxazole. sulphamethoxazole in other Oxford dictionaries Definition of sulphamethoxazole in the US English dictionary