Levonorgestrel, used as emergency contraception, is most effective when taken as soon as possible after unprotected sex and within 72 hours (3 days). Its effectiveness declines with time, so earlier dosing provides better protection. Here are the key points:
- What it is: Levonorgestrel is the active ingredient in many morning-after pills (emergency contraception). It works primarily by delaying ovulation and, to a lesser extent, by affecting the movement of sperm or eggs to prevent fertilization. It does not terminate an established pregnancy and is not effective if implantation has already occurred.
- Time window and effectiveness:
- Within 24 hours: up to about 95% reduction in pregnancy risk in some findings, with other sources reporting around 89–95% depending on data and population. Effectiveness is highest the sooner it is taken.
* Within 25–48 hours: roughly 85% effectiveness in preventing pregnancy in some estimates.
* Within 49–72 hours: effectiveness decreases further, with estimates around 58% in some sources, though real-world effectiveness can vary.
* Overall guidance: it is a backup method and should not be used as a routine form of contraception. It does not protect against sexually transmitted infections.
- Dosing and options:
- Most commonly available as a single-dose tablet (e.g., 1.5 mg levonorgestrel) or in two 0.75 mg tablets in some formulations. It’s designed for use within 72 hours of unprotected sex. Some products may differ slightly in dosing, so follow the package instructions.
- Counseling considerations:
- If you are within the 72-hour window, taking levonorgestrel as soon as possible maximizes effectiveness. If menstrual bleeding is delayed or changes occur, pregnancy testing may be considered after a missed period. If vomiting occurs within 2–3 hours of taking the pill, a repeat dose may be necessary per product guidelines.
* Plan B One-Step is a commonly used formulation and is generally effective within 72 hours; it is not a regular contraceptive and does not substitute for ongoing contraception.
- Special populations and limitations:
- Levonorgestrel is less effective in certain contexts (e.g., very high body weight or BMI in some analyses) and may be less protective in those populations, though data are nuanced. It remains most effective when used promptly after sex.
If you’d like, I can tailor this to your timeframe (how soon after sex you took or plan to take it), discuss alternatives (like copper IUD, which can be more effective for some people if available within a certain window), and provide guidance on next steps and when to consider a pregnancy test.
