What is the failure rate of emergency oral contraception, such as levonorgestrel (Emergency Contraception), in a female patient of reproductive age with no known contraindications?

Failure Rate of Emergency Oral Contraception

Levonorgestrel emergency contraception has a failure rate of approximately 15% (or conversely, an effectiveness rate of up to 85%), while ulipristal acetate demonstrates superior efficacy with a failure rate of approximately 2% when taken within 120 hours of unprotected intercourse. 1, 2

Levonorgestrel-Based Emergency Contraception

Effectiveness varies by timing and patient weight:

• Overall efficacy: Prevents up to 85% of expected pregnancies, translating to a failure rate of approximately 15% 1
• Timing considerations: While traditionally approved for use within 72 hours, effectiveness is highest when taken as soon as possible after unprotected intercourse 1, 3
• Weight-related efficacy reduction: Levonorgestrel may be significantly less effective in women weighing more than 165 pounds (BMI >30 kg/m²), with some guidelines recommending alternative methods for this population 1
• Dosing: Single 1.5 mg dose (either as one pill or two 0.75 mg pills taken together) 1, 4

Ulipristal Acetate (ella)

Superior efficacy, especially in the later time window:

• Failure rate: Approximately 1.9-2.2% when taken within 120 hours of unprotected intercourse 2
• Extended window: Approved for use up to 120 hours (5 days) after unprotected intercourse, with maintained efficacy across all five 24-hour intervals 2, 3
• Weight considerations: For women with BMI >30 kg/m², the observed pregnancy rate was 3.1%, which was not significantly reduced compared to expected rates, though this still represents better performance than levonorgestrel in this population 2
• Comparative advantage: May have greater effectiveness than levonorgestrel, particularly at the end of the 5-day window and in women weighing more than 165 pounds 1

Copper IUD as Emergency Contraception

The most effective emergency contraception option:

• Failure rate: Less than 1% 1
• Time window: Can be inserted up to 7 days after unprotected intercourse 3
• Added benefit: Provides ongoing highly effective contraception for up to 10 years 1
• Not affected by body weight or drug interactions 1

Yuzpe Regimen (Combined Oral Contraceptives)

Less commonly used due to inferior tolerability:

• Failure rate: Approximately 2-3% with perfect use 5
• Side effects: Significantly higher rates of nausea (50%) and vomiting (20%) compared to levonorgestrel 1
• Levonorgestrel is preferred due to superior adverse effect profile while maintaining similar effectiveness 1

Critical Clinical Considerations

Important factors affecting failure rates:

• Timing is crucial: All oral methods are more effective the sooner they are taken after unprotected intercourse, though no significant time trend was demonstrated for ulipristal across the 120-hour window 2, 4
• Mechanism limitation: Emergency contraception primarily works by delaying or inhibiting ovulation; it does not disrupt an established pregnancy 1
• Drug interactions: Levonorgestrel levels may be significantly reduced in women taking efavirenz or other CYP3A4 inducers 1
• BMI >30 kg/m²: Progestin-only emergency contraception pills should be avoided in this population; consider ulipristal or copper IUD instead 1

Practical Recommendation Algorithm

For optimal outcomes, choose emergency contraception based on:

1. First-line for most patients within 72 hours: Levonorgestrel (readily available over-the-counter) 1
2. Preferred if 72-120 hours post-intercourse OR BMI >30 kg/m²: Ulipristal acetate 1, 2
3. Most effective option if acceptable to patient: Copper IUD (up to 7 days post-intercourse) 1
4. Avoid: Yuzpe regimen due to poor tolerability unless other options unavailable 1