Why Emergency Contraception Fails

Emergency contraception (EC) is a critical tool for preventing unintended pregnancy following unprotected intercourse or contraceptive failure. However, despite its widespread availability and use, EC is not 100% effective—and understanding the reasons for failure is essential for informed clinical counseling and patient education.

One of the most significant contributors to EC failure is timing. Levonorgestrel-based pills—such as Plan B One-Step and generic equivalents—are most effective when taken within 24 hours of unprotected sex, with efficacy declining progressively thereafter. By 72 hours post-coitus, effectiveness drops substantially; beyond this window, the risk of ovulation occurring before drug action increases markedly. Ulipristal acetate (e.g., Ella), while more effective at later time points, still loses potency if administered more than 120 hours after exposure.

Another key factor is interference from concurrent medications. Enzyme-inducing agents—including certain antiepileptics (e.g., carbamazepine, phenytoin, topiramate), rifampin, and St. John’s wort—accelerate hepatic metabolism of levonorgestrel and ulipristal acetate, reducing systemic drug exposure and compromising contraceptive effect. Patients on these regimens require alternative strategies, such as copper intrauterine device (IUD) insertion, which remains highly effective up to five days post-intercourse and is unaffected by pharmacokinetic interactions.

Body weight also influences EC performance. Multiple studies indicate reduced efficacy of levonorgestrel in individuals with higher body mass index (BMI), particularly above 25 kg/m², with diminishing returns observed above 30 kg/m². While ulipristal acetate appears less affected by BMI, data remain limited, and the copper IUD remains the gold-standard option for individuals with obesity seeking emergency contraception.

Additionally, EC does not prevent pregnancy once implantation has begun—and it offers no protection against subsequent acts of unprotected intercourse. Misconceptions about its mechanism—such as the erroneous belief that it induces abortion—can lead to inappropriate use or delayed administration. In reality, EC primarily delays or inhibits ovulation; it does not disrupt an established pregnancy or affect implantation in clinically meaningful ways.

Clinicians should emphasize that emergency contraception is not a substitute for consistent, evidence-based primary contraception. Shared decision-making—including discussion of long-acting reversible contraceptives (LARCs) like IUDs and implants—should occur during EC counseling to reduce future risk of unintended pregnancy.