Oral contraceptive failure rates and oral antibiotics

Despite anecdotal evidence of a possibility of decreased effectiveness of oral contraceptives (OCs) with some antibiotics, it is not known whether antibiotic use in dermatologic practices engenders any increased risk of accidental pregnancy.

Our purpose was to examine the effect of commonly prescribed oral antibiotics (tetracyclines, penicillins, cephalosporins) on the failure rate of OCs.

The records from three dermatology practices were reviewed, and 356 patients with a history of combined oral antibiotic/OC use were surveyed retrospectively. Of these patients, 263 also provided "control" data (during the times they used OCs alone). An additional 162 patients provided control data only.

Five pregnancies occurred in 311 woman-years of combined antibiotic/OC exposure (1.6% per year failure rate) compared with 12 pregnancies in 1245 woman-years of exposure (0.96% per year) for the 425 control patients. This difference was not significant (p = 0.4), and the 95% confidence interval on the difference (-0.81, 2.1) ruled out a substantial difference (> 2.1% per year). There was also no significant difference between OC failure rates for the women who provided data under both conditions, nor between the two control groups. All our data groups had failure rates below the 3% or higher per year, which are typically found in the United States.

The difference in failure rates of OCs when taken concurrently with antibiotics commonly used in dermatology versus OC use alone suggests that these antibiotics do not increase the risk of pregnancy. Physicians and patients need to recognize that the expected OC failure rate, regardless of antibiotic use, is at least 1% per year and it is not yet possible to predict in whom OCs may fail.