Plan B Timing After Unprotected Sex
Plan B (levonorgestrel) should be taken as soon as possible within 72 hours (3 days) of unprotected sex, though it can still be used up to 120 hours (5 days) afterward with decreasing effectiveness. 1, 2
Optimal Timing Window
- The sooner you take Plan B, the better it works – this is the single most important factor for effectiveness 3, 2
- The FDA-approved window is within 72 hours (3 days) of unprotected intercourse 2
- CDC guidelines extend this to up to 120 hours (5 days), though efficacy significantly decreases after 72 hours 1
Effectiveness by Time Interval
- Within 24 hours: Highest effectiveness, reducing pregnancy risk by at least 75% 3
- 24-72 hours: Remains effective with similar pregnancy prevention rates 1
- 72-96 hours (days 3-4): Pregnancy rates remain relatively low but begin to increase 1, 3
- 96-120 hours (days 4-5): Effectiveness drops substantially – the odds of pregnancy increase almost 6-fold compared to taking it on day 1 4
A meta-analysis found that pregnancy rates were low when administered within 4 days but increased significantly at 4-5 days after unprotected intercourse 1
Important Clinical Considerations
Dosing Options
- Single 1.5 mg dose (Plan B One-Step) – can be taken all at once 2
- Two 0.75 mg doses – both doses can be taken simultaneously without decreasing efficacy or increasing side effects 3
Reduced Effectiveness Scenarios
- Higher body weight/obesity: Plan B may be less effective in women with higher BMI, though it should still be offered 1, 3
- Certain medications: Efavirenz (HIV medication), rifampin (tuberculosis treatment), or seizure medications may reduce effectiveness 2
After Taking Plan B
- Any regular contraceptive method can be started immediately after using levonorgestrel 1, 3
- Use barrier contraception or abstain for 7 consecutive days while starting regular contraception 1
- If vomiting occurs within 2-3 hours of taking the dose, another dose should be taken as soon as possible 3, 2
- Expect your next period within a week of the expected time; if delayed beyond 1 week, take a pregnancy test 2
Critical Pitfall to Avoid
Do not wait to see if pregnancy occurs – the most common mistake is delaying administration. Every hour counts, particularly after 72 hours when effectiveness drops precipitously 3, 4. Consider advance provision so Plan B is immediately available when needed 1, 3.