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Interactions Between the Birth Control Pill and Antibiotic or Antifungal Therapy
by John Wilks
Released October 12, 2005
Interactions between the birth control pill and antibiotic or antifungal therapy: a brief literature review.
By John Wilks B.Pharm MPS MAACP
Consultant pharmacist
In answer to concerns regarding the veracity of suggested interactions between the birth control pill (BCP) and commonly prescribed antibiotic and antifungal medications, I herein provide a brief review of this topic. This paper is not an exhaustive literature review; rather, it is an indicative and hopefully fair guide on this important question.
Griseofulvin
Griseofulvin is prescribed for a variety of skin, nail and hair infections by susceptible fungal agents. A number of case reports indicated that griseofulvin has can interfere with the action of the BCP. Cote (1990) reported the first case of such an interaction in a healthy 25-year woman taking the BCP who was also prescribed 330mg of ultramicrocrystalline griseofulvin twice daily. Within three months she became pregnant.
Sulphonamides
Sparrow (1989) reported there were 5 case of sulphonamide-induced pill failure in two separate studies she had conducted. In their first study of 163 women there were 3 pregnancies, and in the second study of 137 women there were 2 pregnancies. These pregnancies occurred with cotrimoxazole (a combination of trimethaprin and sulphamethoxazole). Van Dijke and Weber (1984) have also reported on two cases of pregnancy in women taking the BCP, griseofulvin and a sulphonamide.
Tetracycline
Sparrow (1989) reported 3 cases of suspected tetracycline-related pregnancy in women taking the BCP. The author stated the pregnancies occurred in women taking the medication for "acute infections." Women using tetracycline for a long-term medical condition, in conjunction with the BCP were not considered to be likely to conceive, since the bowel flora would be at a stable level. King reported three pregnancies in women taking minocycline.
Cephalosporins, penicillins, erythromycin and metronidazole
In his review paper, Dickinson cites research by the Committee on the Safety of Medicines (United Kingdom) indicating a total of 63 reported unplanned pregnancies between 1968-1984. Penicillins and tetracyclines were involved in 70% of the cases. Metronidazole, cephlosporins, erythromycin, fusidic acid, sulphonamides and trimethoprim were also implicated.
Dickinson also reports that in one study 3 of 25 women appeared to ovulate whilst taking a low dose monophasic BCP and metronidazole (400mg three times a day for 6-8 days).
King (1997) reported on two pregnancies occurring in women taking a cephalosporin.
Rifampacin
There is no debate that rifampcin frequently leads to BCP failure.
Concluding remarks
When studies are conducted to determine the effect of antibiotics on the levels of hormones contained within the BCP, the summary data is frequently pooled. This has the effect of statistically eliminated an individual variations in the blood levels of these hormones. This point is well made by Dickinson, who noted that "except for rifampcin, antibiotics do not significantly affect the plasma concentrations of ethinyl estradiol (or norethindrone and levonorgestrel) based on pooled data; however, large individual variations in plasma concentrations of ethinyl estradiol and norethindrone occurred."
Perhaps the last word on this topic resides within the recently promulgated policy of the American Medical Association.
Policy of the AMA in June 2000
- Women prescribed rifampin concomitantly with OCs faced significant risk of OC failure and should be counselled about the additional use of nonhormonal contraceptive methods during the course of rifampin therapy.
- Women using combined OCs should be informed about the small risk of interactions with antibiotics and that it is not possible to identify in advance the women who may be at risk of OC failure. Women who are not comfortable with the small risk of interaction should be counselled about the additional use of nonhormonal contraceptive methods. Women who have had previous OC failure or who develop breakthrough bleeding during concomitant use of antibiotics and OCs should be counseled about the use of alternate methods of contraception if they engage in intercourse during the period of concomitant use, as they may be part of the subset of women at high risk of contraceptive failure.
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