Mechanism
Antibiotics can potentially reduce the effectiveness of hormonal birth control due to their effects on the liver. According to a 2002 article in the Journal of Gynecologic Surgery, some antibiotics increase the activity of certain enzymes in the liver. One of these enzymes, called cytochrome p450, is involved in breaking down ethinyl estrogen and progesterone within the body. Increased activity of cytochrome p450 can cause failure of hormonal birth control because this form of contraception depends on there being steady levels of estrogen and/or progesterone to prevent ovulation. Thus, antibiotics could, in theory, cause hormonal birth control to be less effective.
Antibiotics and Cytochrome p450
The liver works to process drugs and hormones from the body by breaking them down and making it easier for the body to excrete them via the urine. The liver not only breaks down naturally occurring hormones in the body, but also those found in hormonal birth control. As some antibiotics are broken down, they cause an increase in the liver's activity. As the Mayo Clinic notes, rifampin (which is often used to treat tuberculosis) is known to increase liver enzymatic activity to the point where it could interfere with birth control. Another medication, the anti-fungal compound griseofulvin, can also increase cytochrome p450 activity. Other antibiotics can also potentially increase the activity of these liver enzymes, although to a lesser extent. Increased activity of the liver enzymes causes the hormones in the birth control to be removed from the body more rapidly.
Pill Metabolism and Decreased Ability
When the hormones in birth control are eliminated from the body faster, it causes their levels to drop. Hormonal birth control works by keeping a steady amount of estrogen and/or progesterone in the body. As the American Experience (from PBS) explains, this causes suppression of luteinizing hormones and follicle-stimulating hormones, which are both needed for ovulation (and thus, fertility) to occur. If estrogen and progesterone levels dip due to antibiotic use, this can cause the levels of those other two hormones to rise. If this happens, it is possible that a mature egg will be released, making pregnancy possible. Thus, increased activity of the liver due to antibiotic use can remove the birth control hormones out of the body faster, resulting in a loss of the hormonal block of ovulation.
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