I have been on Apri 28-day birth control
pills for over a year. I take the pill
on time, as directed, every month. Last Sunday I began taking Sulfamethoxazole-TMP (generic Bactrim DS) to treat a urinary tract infection. I have continued to take my birth control pills every day on schedule, but I was advised to use a back up method of birth control, as is standard. The day that I began taking the antibiotics was day 16 of my cycle (using the first day of my last period as cycle day 1).