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 why this occurs and how much you need to worry. Understanding this potential interaction requires a little background on how drugs are metabolized in your body and absorbed into your system.
Most people think metabolism is the reason why their skinny friend can subsist on cheeseburgers and french fries without gaining weight. That is partly right. Metabolism is how your body breaks down the things you ingest into substances that it can use. Drugs are metabolized just like food. In fact, very few drugs are absorbed by your body unchanged. Most of them are metabolized by the liver or the kidneys into different chemical compounds. This is called first pass metabolism.
The estrogen found in birth control pills is metabolized by the liver. Ethinyl estradiol, the synthetic estrogen most birth control pills contain, has a bioavailability of 40 to 50 percent. This means that only about half of the drug is absorbed into your bloodstream unchanged.
Once the estrogen in birth control pills is metabolized by the liver, it is secreted into the intestine along with the bile that the liver manufactures. Rather than passing through the digestive tract, however, the metabolized estrogen interacts with the normal flora, or good bacteria, that live in the intestines. The normal flora change this metabolized estrogen back into a form that can be used by your body. This happens over and over again and helps keep estrogen levels up.
Antibiotics and Normal Flora
The purpose of antibiotics is to kill the bacteria that are making you sick. Unfortunately, antibiotics don't discriminate between good and bad bacteria. This is why antibiotics often cause diarrhea and yeast infections: They kill off the good bacteria that keep your system in balance. They can also kill off the good bacteria that keep the synthetic estrogen in birth control pills recirculating in your system.
Experts rarely agree, and there is a lot of controversy on the significance of the interaction between birth control pills and antibiotics. Amoxicillin is said to interact with birth control pills 1 percent of the time. That is a small number, but for most women, an unplanned pregnancy is a significant event. Most doctors recommend using a back-up method of birth control while you're taking antibiotics and for seven days after the antibiotic course is complete, just to be safe.
- Dental Clinics of North America: Antibiotics and Oral Contraceptives
- Principles and Practice of Pediatric Infectious Diseases; Sarah S. Long; 2008
- "Medical Clinics of North America;" Antibiotic Drug Interactions; Pai, Momary, and Rodvold; November 2006