Simultaneous antibiotic, pill use reviewed. (News Briefs).

A small number of women who use combined oral contraceptives (COCs) and antibiotics at the same time may be at risk for pregnancy, an extensive review of studies of the impact of antibiotic treatment on oral contraceptive effectiveness has shown.

Because such women cannot be identified in advance, the researchers who conducted the review urge providers to use caution when prescribing COCs to women using antibiotics. Such women should be told about the risk of COC interactions with antibiotics and the impossibility of identifying who may be at risk of COC failure. Women on short- or long-term broad-spectrum antibiotic therapy who are uncomfortable with the potential risk of a drug interaction should be counseled to use another nonhormonal contraceptive method, the reviewers recommended. Women who have had previous COC failures or who develop breakthrough bleeding while using both antibiotics and COCs should be counseled to use other contraceptive methods. However, the researchers advised against changing the contraceptive method of women on short-term antibiotic therapy. Such a change could reduce contraceptive compliance and efficacy.

(Notably, the World Health Organization medical eligibility criteria caution only that women taking specific antibiotics that increase the rate of hepatic drug metabolism, such as rifampicin and griseofulvin, avoid using COCs.)

The review of previous studies revealed that, with the exception of rifampin (which clearly impaired the effectiveness of oral contraceptives), oral antibiotics did not significantly affect in most women the metabolism, or blood concentrations, of oral contraceptive hormones that prevent pregnancy. However, some individuals using both oral contraceptives and certain oral antibiotics, such as tetracycline and penicillin derivatives, had significant decreases in the concentration of the contraceptive hormone ethinyl estradiol in their blood and appeared to ovulate, putting them at some risk of pregnancy.

"The progestins in COCs do not appear to be affected by these antibiotics and, thus, could be expected to provide reliable contraception analogous to a mini-pill, even if estrogen levels are reduced," notes Dr. David Grimes, vice president of biomedical affairs at FHI. "However, as is true of estrogen, the level of progestins in the blood of various women of similar body mass taking the same COC varies widely. Furthermore, progestin-only pills must be taken at the same time each day to work best."

The review conducted by the American Medical Association (AMA)'s Council on Scientific Affairs, was published in the November 2001 issue of Obstetrics & Gynecology. The researchers' recommendations have been adopted as AMA policy.