Question: Will antibiotics make my birth control pill less effective?

Answer: There has been much discussion and controversy concerning the interaction of antibiotics and birth control pills. In the past it was thought that taking birth control pills with ANY antibiotic would lower the effectiveness of the contraceptive. This belief arose after women reported unintended pregnancies while on both birth control pills and antibiotics.

The limited number of studies have focused on determining whether taking a commonly prescribed antibiotic while on an oral contraceptive puts women at a higher risk of getting pregnant. Birth control pills, like all drugs, are not 100% effective. The pregnancy rate during these studies was within the normal "typical use" failure rate for birth control pills. This suggests that there is not an increased risk of birth control failure while taking an antibiotic.

Currently, experts believe the overall risk is small and that some antibiotics interfere with birth control pills more than others. The interaction may be exaggerated, although, two antibiotics are definitively known to reduce the effectiveness of the birth control pill. These are rifampin (Rifadin, Rimactane), used to treat tuberculosis and griseofulvin (Gris-PEG), used to treat some fungal infections.

Rifampin and griseofulvin interact by speeding up metabolism of the pill by increasing activity of certain enzymes in the liver. The blood levels of the hormones decrease and the birth control pill becomes less effective.

Other antibiotics can decrease the bacteria normally found in the intestines of healthy people. These bacteria help break down estrogen compounds and release active hormone which is then reabsorbed. Without these bacteria, there may be less active hormone in the bloodstream for the body to use.

The risk of getting pregnant when taking antibiotics while on birth control pills is small, but there is a risk. The incidence of pregnancy in women who take antibiotics while on birth control pills is unknown. Use a back-up or alternative method of contraception (condom or diaphragm) if you are prescribed rifampin or griseofulvan. More research is needed to determine the increased risk of pregnancy with other antibiotics, but to be safe you can use a second method of birth control if you are not comfortable with the risk of getting pregnant.

Talk to your doctor or pharmacist about all of the medications you are on and any concerns you may have.

Last Updated: November 24, 2013