Antibiotics might decrease the effectiveness of birth control pills (contraceptives). Birth control pills are composed of hormones that prevent the female reproductive system from preparing for insemination of the sperm.
There have been reports of pregnancy in women who have taken antibiotics while using birth control pills. The relationship has not been adequately studied to date. This may be in part because it is difficult getting volunteer women to take the pill with antibiotics to see if pregnancy occurs. Theoretically, additional medications might alter the metabolism and blood levels of the birth control hormones, thereby altering their effectiveness.
Nevertheless, it is noteworthy that antibiotics, such as rifampin, ampicillin, tetracylines, and griseofulvin have been reported to cause breakthrough bleeding and menstrual irregularities in women taking birth control pills. Similar effects have also been reported in women taking the seizure medications phenytoin and barbiturates.
It is also noteworthy that birth control pills are not 100 percent effective anyway.
It would be advisable for women who are using birth control pills to discuss with their doctors the use of additional contraceptive measures while taking antibiotics for optimal prevention of pregnancy.