Bactrim is especially effective against numerous complicated skin infections like boils (skin abscess), cellulitis (red and painful skin rash in particular areas), folliculitis (infection at the hair follicles), impetigo (hey-colored blisters on the skin), paronychia (infection on the skin around nails). Bactrim effectively kills methicillin-resistant Staphylococcus aureus (MRSA) bacteria, which is resistant to various antibiotics.