I started taking antibiotics on Saturday the 23 for a kidney infection. They gave me Bactrim to take twice a day for ten days. I went back for a check up while still on antibiotics and they tested my urine and said the infection had completely cleared, and that i can stop taking the antibiotics. I understand that if you are on birth control antibiotics make the birth control ineffective for a week after you stop taking them. But what if I'm about to start taking birth control? If I stop taking antibiotics today, the 28th, one week would be the 5th of May. My period is scheduled to start on the 3rd. If I start taking the birth control the day my period starts will it still be effective immediately like the instructions say? Or Will the antibiotics effect that?