Plan B (Levonorgestrel) Emergency Contraception
Plan B should be taken as a single 1.5 mg dose of levonorgestrel as soon as possible within 5 days (120 hours) of unprotected sexual intercourse, though it is most effective when taken immediately. 1
Dosing Options
Two equivalent dosing regimens are available for levonorgestrel emergency contraception:
- Single dose: 1.5 mg levonorgestrel taken once (preferred for simplicity and compliance) 1, 2
- Split dose: 0.75 mg levonorgestrel followed by a second 0.75 mg dose 12 hours later (equally effective but less convenient) 1, 2
The single-dose regimen is now the standard recommendation as it provides equivalent efficacy with better adherence and no need to remember a second dose 2.
Timing and Effectiveness
- Take as soon as possible after unprotected intercourse—every hour of delay reduces effectiveness 1
- Effective up to 5 days (120 hours) post-intercourse, though pregnancy prevention rates decline after 72 hours 1
- Within 3 days: levonorgestrel prevents approximately 80% of expected pregnancies 3
- Days 3-5: effectiveness decreases but still provides meaningful protection 1
Mechanism and Important Caveats
Levonorgestrel works primarily by inhibiting or delaying ovulation when taken before the LH surge—it does NOT work if ovulation has already occurred 3, 4. This is why timing is critical.
Critical Limitations:
- Levonorgestrel may be less effective in obese women—consider ulipristal acetate (30 mg) or copper IUD as superior alternatives in this population 1
- Ulipristal acetate is MORE effective than levonorgestrel when taken 3-5 days after intercourse 1
- The copper IUD is the MOST effective emergency contraception option (>99% effective) and can be inserted up to 5 days after unprotected intercourse 1
After Taking Plan B
Starting Regular Contraception:
- Can start combined oral contraceptives immediately after taking emergency contraception 5
- MUST use backup contraception (condoms) for 7 consecutive days after starting regular birth control pills 5
- It takes 7 days of consistent pill use to suppress ovulation adequately 5
Expected Side Effects:
- Nausea, vomiting, headache, breast tenderness 3
- Menstrual cycle disruption is common—next period may be early, late, heavier, or lighter 3
- If vomiting occurs within 2 hours of taking the dose, consider repeating the dose 3
Common Pitfalls to Avoid
- Do NOT delay taking Plan B to wait for a "better time"—immediate administration maximizes effectiveness 1
- Do NOT rely on Plan B as regular contraception—it is significantly less effective than ongoing contraceptive methods 3
- Do NOT assume protection if taken after ovulation has occurred—the medication cannot prevent pregnancy once ovulation is complete 3, 4
- Do NOT skip backup contraception when starting regular birth control after emergency contraception—pregnancy risk remains high during the first 7 days 5
When to Consider Alternatives
Consider ulipristal acetate (30 mg) or copper IUD instead of levonorgestrel if: