Clinical pearls: factors affecting reported contraceptive efficacy rates in clinical studies

Current combination oral contraceptives (OCs) are among the most popular, safe, and effective methods of reversible contraception. There are, however, many factors that can affect contraceptive failure rates as reported in clinical trials, including subject characteristics, factors related to study methodology and data analysis, and publication biases. The variability of these factors among clinical trials makes meaningful comparisons of contraceptive efficacy data across studies difficult, if not misleading or erroneous. It is even more difficult to reconcile the differences between clinical trial efficacy rates and everyday use rates; for instance, the National Survey of Family Growth reported that the rate of OC failure is close to 8% in the United States, which is higher than rates reported in clinical trials. Thus, it is important for the clinician to consider the many factors that can influence reporting of contraceptive failure rates in clinical trials and be aware of the limitations in differentiating OCs on the basis of contraceptive efficacy derived from clinical trial data. Furthermore, clinical trial data may not accurately predict contraceptive efficacy in everyday use.