If you rely on oral contraceptives for birth control, chances are you have heard that antibiotics like amoxicillin can decrease their effectiveness. You may be wondering how this can occur and if it is true. The issue is based on the possibility that amoxicillin may decrease the amount of ethinyl estradiol -- a common estrogen in oral contraceptive pills -- in your body. There is little to no good scientific evidence that amoxicillin will reduce the effectiveness of these pills, but if you want to make sure that you do not become pregnant, it would be wise to use another form of birth control while taking amoxicillin and for 1 week afterwards.
Metabolism and Recirculation of Oral Contraceptives
When you take an oral contraceptive containing ethinyl estradiol, this estrogen is absorbed from the intestines into the blood stream and then travels to the liver. About half of the ethinyl estradiol will pass through the liver unchanged and travel to the rest of the body to exert its contraceptive effects. The other half is metabolized in the liver to inactive forms. One of the inactive forms travels to the kidneys, where it leaves the body in the urine. The other inactive form, which has a chemical group called glucuronide attached to it, is sent through the bile ducts back to the intestines. Bacteria naturally living in the intestines have the ability to remove glucuronide, allowing ethinyl estradiol to be absorbed back into the body. This causes recirculation of the hormone, a process known as enterohepatic cycling.
Antibiotics and Normal Intestinal Bacteria
The purpose of antibiotics is to kill bacteria that are causing an infection. Unfortunately, antibiotics don't discriminate between good and bad bacteria. Thus, when antibiotics are taken orally, they can reduce the number of beneficial bacteria living in the intestines. As this includes the bacteria that remove glucuronide from ethinyl estradiol, antibiotics can reduce the amount of ethinyl estradiol that is reabsorbed back into the body.
Amoxicillin and Oral Contraceptive Levels
The effectiveness of oral contraceptives relies on adequate levels of ethinyl estradiol in the body, so the question arises whether oral amoxicillin can interfere with enterohepatic cycling of the estrogen enough to reduce these levels. The answer is unclear, as little research has been conducted about this issue for amoxicillin. A study published in the January 1997 issue of “Contraception” reported no effect of oral amoxicillin on ethinyl estradiol levels, but this was performed in rabbits. For ampicillin, a close relative of amoxicillin, initial studies reported that ampicillin lowered blood levels of ethinyl estradiol, but later studies showed that it had no effect on these levels, as discussed in a review article published in the November to December 1999 issue of “The Canadian Journal of Infectious Diseases.”
The Birth Control Question
According to the 1999 “The Canadian Journal of Infectious Diseases” article, amoxicillin is one of a group of antibiotics that have been associated with failure of oral contraceptive birth control in a few isolated case reports, but which have not been shown to reduce contraceptive effectiveness in any proper scientific study. Guidelines published by the American Society of Gynecologists in June 2006 indicated that there is no scientific evidence that any antibiotic except rifampin can decrease the effectiveness of oral contraceptives. However, the labeling information for amoxicillin, as required by the Food and Drug Administration, indicates that it may affect intestinal bacteria, like other antibiotics, leading to reduced estrogen reabsorption and decreased effectiveness of oral contraceptives containing estrogen. So if you definitely want to avoid becoming pregnant, it would be best to use a back-up method of birth control the whole time you are taking amoxicillin and for 7 days afterwards.
Reviewed and revised by: Mary D. Daley, M.D.