Is keflex effective for boils?

Not very: By the time someone has a boil, it's generally too late for antibiotics. A boil needs to be opened and drained. In addition, a large proportion of boils or abscesses these days are caused by mrsa, which can't be treated with keflex. Mrsa is generally treated with Bactrim (sulfamethoxazole and trimethoprim) or clindamycin, but again, drainage is the key.