Bactrim ds (trimethoprim + sulphamethoxazole) is a combinational antibiotic. Bactrim is usually not preferred for treatment of sinusitis but can be preferred if the bacteria is known to be susceptible for Bactrim. I would have started my patient on amoxicillin clavulanate or doxycycline if allergic to penicillin. The choice of antibiotics also depends upon the local susceptible patterns. So Bactrim can also be effective in some regions. Hope the above information solves your query.