Emmanue asked
My girlfriend just finished taking doxycycline for kidney infection 5 days ago. Would her birth control still be affected by the antibiotic?
Answer
There is some controversy regarding whether or not antibiotics, like doxycycline, can make birth control pills less effective. Most large studies show that the hormone levels from birth control pills are not lowered by most antibiotics, including doxycycine and backup is usually not needed. Based on available data, classes of antibiotics that are not thought to reduce the efficacy of birth control pills include:
- Macrolides (e.g. clarithromycin)
- Penicillin (conflicting evidence - see below)
- Cephalosporins (e.g. cephalexin)
- Quinolones (e.g. ciprofloxacin)
- Metronidazole
- Doxycycline (conflicting evidence - see below)
- Tetracycline
However, there is one class of antibiotics that are known to decrease birth control hormone concentrations, the "rifamycin" class of antibiotics. These speed up the metabolism of estrogen, causing lower concentrations and in some cases, lower the effectiveness of birth control.
What Is The Theory Behind Antibiotics And Birth Control?
Antibiotics, like doxycycline, historically were thought to decrease bacteria in the GI tract, which could interrupt the enterohepatic recirculation of estrogen (i.e. bacterial conversion of estrogen metabolites back into ethinyl estradiol, which is reabsorbed by the body). Specifically, the interaction is thought to work like this:
- The estrogen (ethinyl estradiol) in oral contraceptives is primarily metabolized in the liver to the metabolite 2-hydroxy-ethinylestradiol and other hydroxylated and methylated metabolites.
- These estrogen metabolites undergo what is known as enterohepatic recirculation. This means that these estrogen metabolites are recirculated into the GI tract after being processed by the liver. The bacteria in our GI tract can change the metabolized estrogen back into the active, parent compound, ethinyl estradiol, which increases overall absorption and concentrations of estrogen.
- Antibiotics, like doxycycline, theoretically eliminate much of our normal GI flora. Therefore, less of the metabolized estrogen is able to be converted back into the active compound, reducing overall concentrations and protection from pregnancy.
Most studies suggest there is little evidence that interruption of this re-absorption affects contraceptive efficacy.
Why back-up contraception is generally recommended for those taking birth control pills with antibiotics is because some studies show a small number of individual patients do show large decreases in the plasma concentrations of ethinyl estradiol (the estrogen in birth control pills). These individuals are difficult, if not impossible, to identify in advance. Therefore, a cautious approach is prudent, as the consequences of unintended pregnancy are great. Per one study:
In pooled results obtained from relatively small populations, oral antibiotics, with the exception of rifampin, have not significantly affected the pharmacokinetics of ethinyl estradiol, levonorgestrel, and norethindrone or reduced the serum concentrations of gonadotropins. However, individual patients have been identified who experienced significant decreases in the plasma concentration of these components of OCs and who appeared to ovulate.
In the study that supplied the quote above, it specifaclly noted that the rare decreases in hormone concentrations came with penicillin and tetracycline derivative (e.g. doxycycline) antibiotics.
Another concern with antibiotic use in those taking birth control pills is nausea that causes vomiting or diarrhea, common side effects from many antibiotics. Vomiting or diarrhea can reduce the absorption of the hormones from birth control pills, potentially reducing how well they work. In general, if a woman on a birth control pills has vomiting or diarrhea for more than 2 days, it is recommend to use backup birth control until it's over and she's taken 7 days of active pills
Recommendations For Those Taking Antibiotics And Birth Control Pills
If you are not taking a "rifamycin" antibiotic (which doxycycline is not), your birth control will most likely not be affected. However, since there do appear to be a few individuals that are especially susceptible to antibiotics decreasing birth control hormone concentrations, and since these individuals can't be easily identified, it is generally recommended to use back-up contraception while taking doxycycline, just to be safe.
In terms of how long to wait after taking antibiotics to stop back-up contraception, the most common recommendation is for 7 days after you finish your antibiotic. This allows time for the bacteria in your gut to "re-grow" and for hormone concentrations from the birth control pills to return to a normal level.
