Drugs commonly interact with one another. These interactions can be synergistic, compounding the effects, or antagonistic, canceling each others effects. Antibiotics and oral contraceptives, the most commonly used type of birth control, can interact with one another in such a way where their concomitant use causes the contraceptive to fail. When an oral contraceptive "fails", a woman's protection from pregnancy becomes compromised.
The first antibiotic, penicillin, was discovered from a naturally occurring fungus in the 1940s. Since then many other classes of antibiotics have been discovered in nature and synthesized in the laboratory. Antibiotics are a very important drug category. Their ability to limit the damage that a bacterial infection can have on the body prevents severe complications from infections and in some cases, death. Antibiotics kill bacteria by interfering with their structure or metabolism, according to Susan M. Turley, author of "Understanding Pharmacology for Health Professionals". The antibiotic classes that may put a woman at risk for oral contraceptive failure are the penicillin, tetracycline and rifampin.
There are many types of birth control. Some methods are physical, providing a barrier between the mixing of male and female sexual secretions, while others are chemical and affect a woman's menstrual cycle. The chemical methods rely on synthetic hormones, like synthetic estrogens and progesterones. Many delivery systems have been designed to carry these hormones into the body, notes Turley. There are injections, temporary inserts and, the most commonly used category of all, oral contraceptive pills, otherwise known as birth control pills. Oral contraceptive pills, according to a 2000 "Archives of Family Medicine" journal article, are associated with providing reduced protection from pregnancy when taken with penicillin and tetracycline antibiotics.
Oral contraceptives are most commonly, a combination prescription of synthetic estrogens, called ethinyl estradiol and synthetic progesterones, collectively called progestins. The pills are provided based on number of days considered to complete a normal menstrual cycle--28. Turley writes that although 28 pills are provided, only 21 are considered active and actually contain the hormones. The other 7 pills are "sugar pills," otherwise known as placebo, which act as place holders and so that the absence of hormones can simulate the trigger that would normally cause the uterine lining to shed and begin the process of menstruation. However, there are some OCP packs that contain only 21 pills; you should take a week off before starting a new pack.
There are several categories of OCP that are designated based on the contrasting amounts of estrogens and progesterones. Turley lists them as monophasic, biphasic and triphasic. There has not been any specifications provided as to whether any of these categories are less or more affected by their combined use with antibiotics.
The proposed mechanism of action behind the interaction of these two commonly used drugs is revealed in a 2003 "Pharmacy Times" article. Estrogens, whether natural or synthetic, are broken down in the liver and disposed of in the feces. Normal bacteria found in the large intestines can cause the metabolized estrogens from the liver to be diverted from their excretion pathway and brought back into the circulation for activity. The "Pharmacy Times" article suggests that because antibiotics reduce both abnormal and normal bacteria levels, they can reduce the amounts of estrogens being re-circulated, lowering the concentration of hormones necessary to create the contraceptive effect. Also, because antibiotics can cause diarrhea and vomiting, they can cause the OCP to be malabsorbed. The last theory of reduced efficacy suggests that OCP and antibiotics are broken down similarly by the liver, thereby reducing the activity and thus the level of protection that the OCP can provide.
The concern for OCP failure with antibiotic use is prudent because both drugs are used in women of childbearing age. A 2001 "Obstetrics & Gynecology" journal article notes that approximately 10 million women in the U.S. use OCPs annually, and approximately 20 percent of pregnant women note having used antibiotics while on birth control pills. The article notes that women who require antibiotic therapy while using the pill back up their contraceptive attempts with other types of birth control while taking the antibiotic and also in the 7 days following the course.