The effect of antibiotics on birth control pills has been debated over the years. Most scientific studies looking at this question are small and poorly controlled, with conflicting results. Because the type and doses of both oral contraceptives and antibiotics has also changed drastically over the years, early studies may not be relevant to today's prescribed medications.
How could antibiotics cause the Pill to fail?
Early scientific studies suggested that the use of antibiotics (such as penicillin or tetracyclines) for several weeks while taking the oral birth control pill could cause contraceptive failure. The oral contraceptive pill works by elevating blood levels of steroids (primarily estrogen), which in turn block other hormones that are responsible for development and ovulation of an egg every month. If no egg develops, no fertilization and pregnancy can occur. Antibiotics can decrease the intestinal absorption of steroids in the Pill by killing the good intestinal bacteria, which are critical for intestinal absorption. Reduced absorption means lower steroid blood levels, which means less effective disruption of egg production. Some antibiotics can increase the activity of liver enzymes so that the body clears the steroids from the blood stream too rapidly. All of these unintended side effects of antibiotics could reduce the effectiveness of the Pill.
Large scale studies are lacking
Doctors Kevin Burroughs and M. Lee Chambliss reviewed studies performed between 1966 to 2000 examining the possible role of antibiotics in contraceptive failure and published their findings in a series entitled "Solutions to often-asked problems" in the Archives of Family Medicine. Their review concluded that large scale carefully controlled studies do not exist to confirm the observations of smaller studies and anecdotal reports that antibiotics are responsible for oral contraceptive pill failure. Only one antibiotic, rifampicin, prescribed for the treatment of tuberculosis, was convincingly shown to decrease the effectiveness of oral birth control pills.
Better safe than sorry
Even though studies are lacking, most doctors still advise their patients to use another form of birth control when on antibiotics. The reason for this is two-fold. First, some women may be particularly susceptible to the effects of antibiotics on steroid absorption and clearance. Second, the majority of studies were done years ago when much higher dose birth control pills were used. The currently extremely low-dose birth control pills may be more susceptible to failure due to reduced absorption or increased clearance from the body.
Member Comments