When taking Birth Control Pills and antibiotics, it is generally not necessary to use any form of back-up contraception.

Based primarily on anecdotal evidence and theory, some researchers believed that women on BCPs who were taking antibiotics would have small but measurable reductions in their blood hormone levels. These researchers cautioned against relying on oral contraceptives while taking antibiotics.

In controlled studies, this theory has not been proven, and in the case of tetracycline and chlortetracycline, no increased risk of pregnancy was found. Further, careful studies of blood levels of steroid hormones show no change among women taking ampicillin, ciprofloxacin, clarithromycin, doxycycline, metronidazole, ofloxacin, roxithromycin, temafloxacin, and tetracycline.

The only antibiotic demonstrated to have any effect on oral contraceptive effectiveness is rifampin, and it is not clear clinically to what extent, if any, the OCP can be relied upon among women taking this antibiotic.

So with the exception of rifampin, antibiotics are not generally believed to have any impact on the effectiveness of oral contraceptives.