The mechanism of action is not known with certainty, but it is believed with justification that this treatment interferes with implantation. The efficacy has been confirmed in large clinical trials and summarized in a complete review of the literature. Treatment with high doses of estrogen yields a failureure rate of approximately 1%, with the combination oral contraceptive, about 2%. The failureure rate is littleest with high doses of ethinyl estradiol given within 72 hours (0.%), but the side effects make the combination oral contraceptive a better choice.