Taking the morning-after pill at hour 4 versus hour 70 can swing effectiveness from roughly 95% to under 60%, making hours-since-intercourse the single biggest predictor of whether it will work. Two main pill types exist: levonorgestrel-based options like Plan B One-Step work best within 72 hours but lose potency every hour, while ulipristal acetate (ella) holds effectiveness through 120 hours and can still act after the body’s luteinizing hormone (LH) surge begins. You get a time-stamped decision tool covering every hour bracket, the body weight and cycle-position variables that change the math, and how to handle a second need in the same cycle.
This guide covers hour-by-hour effectiveness, when a copper IUD becomes your strongest option, what’s safe, what isn’t, and what to do next if something goes wrong.
The Two Pills and the Windows They Actually Cover
Most morning-after pills sold in U.S. pharmacies contain levonorgestrel, a synthetic progestin that primarily delays or prevents ovulation. Plan B One-Step became available over the counter without age or ID requirements in 2013 and works by stalling ovulation long enough for sperm to die off before an egg is released. At 0–24 hours after unprotected intercourse, levonorgestrel reduces pregnancy risk by roughly 85–95%, and earlier dosing sits at the top of that range.
Ella contains ulipristal acetate, a selective progesterone receptor modulator that can inhibit ovulation even after the LH surge has begun. That distinction matters because once the LH surge starts, levonorgestrel essentially loses its mechanism of action. Ulipristal acetate requires a prescription in the United States for any age, but it remains effective through the full 120-hour window, while levonorgestrel drops off sharply after 72 hours.
Why the 72-hour rule is a midpoint, not a cliff
Levonorgestrel’s original clinical trials simply stopped measuring at the three-day mark, which is why “72 hours” became shorthand for the drug’s window rather than a hard biological cliff. Effectiveness at hour 71 is meaningfully higher than at hour 73, and the decline continues steadily rather than stopping suddenly. Ulipristal acetate was studied through 120 hours (5 days) and shows a much flatter effectiveness curve across that range. That aligns with guidance from the American College of Obstetricians and Gynecologists (ACOG) and the World Health Organization, which both recognize ulipristal as the more forgiving option past 72 hours.
| Feature | Levonorgestrel (Plan B One-Step) | Ulipristal Acetate (ella) |
|---|---|---|
| Optimal timing | 0–72 hours | 0–120 hours |
| Access in U.S. | Over the counter, no age limit | Prescription required |
| Works after LH surge | No | Yes |
| Effectiveness at 24 hrs | ~95% | ~95% |
| Effectiveness at 72 hrs | ~58–60% | ~85% |
| Effectiveness at 96+ hrs | Not recommended | ~75–80% |
How Effectiveness Drops Hour by Hour After Intercourse
Pregnancy risk after a single act of unprotected sex is not constant. It peaks during the fertile window, the 5–6 days leading up to and including ovulation, and drops sharply outside that window. Emergency contraception timing has to be mapped against that biology, not against the clock alone. The faster the pill is taken, the more likely it is to hit before ovulation is triggered.
0–24 hours: peak efficacy for both pill types
At this stage, both levonorgestrel and ulipristal acetate reduce pregnancy risk by approximately 85–95%. Sperm is still in transit, and the LH surge has typically not yet begun. Obtaining a pill within the first 12 hours is the best-case scenario for either option.
24–48 hours: levonorgestrel begins to lose ground
Effectiveness for levonorgestrel drops into the 70–80% range. Ulipristal acetate holds close to its initial efficacy because it can still block the LH surge even if it has started. This is the window where the choice between the two pills starts to matter most.
48–72 hours: levonorgestrel is still useful but less reliable
By hour 60, levonorgestrel’s effectiveness has fallen to roughly 60–65%. Ulipristal acetate remains near 85%. Inside this window with quick prescription access, the switch is worth it. Many telehealth services can issue an ella prescription within an hour.
72–96 and 96–120 hours: ulipristal is the only pill option
Beyond 72 hours, levonorgestrel is essentially off the table, and the absolute pregnancy risk begins climbing meaningfully. Ulipristal acetate is the only oral option that retains meaningful effectiveness, tapering to roughly 75–80% efficacy at 96 hours and around 70% at 120 hours. At this stage, the copper IUD enters the picture as the most effective emergency option available, with a failure rate under 0.1% when inserted within 5 days.
Body Weight, BMI, and Other Variables That Change the Calculus
Body weight and BMI are among the most under-discussed factors in emergency contraception timing. Levonorgestrel efficacy drops measurably in individuals weighing over 75 kg (about 165 lbs) or with a BMI above 30, with some studies suggesting near-zero effectiveness above 80 kg. Ulipristal acetate shows less weight-related decline but still has upper limits worth knowing.
Both drugs are fat-soluble and distribute into adipose tissue, which can lower the peak blood concentration needed to block ovulation. With weight in the higher range, ulipristal acetate is the stronger oral choice, and the copper IUD becomes especially important as a non-weight-dependent alternative.
Cycle position matters more than most people realize
A woman’s cycle day quietly decides whether the pill blocks ovulation, scrambles fertilization, or does nothing at all. If intercourse happened well before ovulation (day 1–10 of a typical 28-day cycle), the window for the pill to act is wide. If it happened the day before or the day of ovulation, you sit in the highest-risk bracket, and the pill must be taken immediately. If ovulation has already occurred, neither oral pill is likely to work, because fertilization may have already taken place. The copper IUD is then the only reliable option, since it prevents fertilization rather than blocking ovulation.
The copper IUD as the strongest emergency option
Inserted within five days of unprotected intercourse, the copper IUD outperforms every oral option and drops failure rates below 0.1%. It reduces pregnancy risk by more than 99%, regardless of body weight or cycle position.
The copper IUD works by releasing copper ions that impair sperm and egg function, preventing fertilization. It can remain in place for up to 10 years as ongoing contraception. The trade-off is the clinic visit, which can be hard to arrange on a weekend or within a tight window. Planned Parenthood and many urgent-care gynecology clinics offer same-day emergency IUD insertion.
Because fast IUD access often hinges on weekend clinic hours, several patient factors quietly determine whether the copper IUD remains a realistic choice.
Using Emergency Contraception a Second Time in the Same Cycle
One of the most common panic-driven questions is whether a second dose in the same cycle is safe or whether it will stop working. A second dose is medically safe, and its effectiveness depends on cycle position, not on prior use. What changes is the rationale. Emergency contraception is not designed for frequent use because it is less effective, more expensive, and produces more side effects than ongoing hormonal methods.
Pill choice for the second event
If the first dose was levonorgestrel and the second incident occurs more than 24 hours later or closer to ovulation, switching to ulipristal acetate for the second dose can improve odds. The two drugs work through similar but not identical pathways, and ulipristal’s ability to block the LH surge gives it an edge later in the cycle.
Common myths about repeated use
- Fertility damage: No credible evidence shows that emergency contraception damages future fertility, no matter how often it’s taken.
- Built-up hormones: The hormones clear the body within days; they do not accumulate.
- Reduced future response: Each dose works independently based on cycle position at the time of dosing.
- Need to “wait” between doses: No medically required waiting period exists, though spacing doses reduces side-effect burden.
What to do if a third or fourth incident occurs within weeks
At that point, emergency contraception is being used as a primary method, and it’s the wrong tool. Transition to a reliable ongoing option: a hormonal IUD, the pill, patch, ring, implant, or the copper IUD if not already in place. ACOG recommends a same-visit IUD insertion whenever possible, because the window for emergency effectiveness and the start of ongoing contraception can be combined into one appointment.
What to Do If the Pill Comes Back Up or Something Goes Wrong
Vomiting within two hours of taking either pill means the dose may not have been fully absorbed. The standard guidance is to redose the same pill as soon as possible, or book a copper IUD insertion within the original 120-hour window if you’d rather not gamble on absorption twice. If vomiting continues and prevents redosing, the IUD becomes the best path forward.
Telling side effects apart from early pregnancy symptoms
Nausea, fatigue, breast tenderness, and light spotting are common after emergency contraception and usually resolve within a few days. These overlap significantly with early pregnancy symptoms, which is why a test is the only reliable way to know. Severe abdominal pain, heavy bleeding, or shoulder pain warrants a same-day clinical evaluation to rule out ectopic pregnancy, which is rare but more likely after any contraceptive failure.
When to take a pregnancy test after emergency contraception
For reliable results, wait roughly three weeks after the unprotected intercourse. Testing earlier produces more false negatives because hCG (human chorionic gonadotropin) may not yet be detectable. If your period is more than a week late, test again or see a clinician for a blood test, which is more sensitive.
Backup contraception after the pill
Use a barrier method (condoms) for the next 7 days after levonorgestrel and 14 days after ulipristal acetate, because the pill can delay ovulation and any subsequent intercourse during that delay carries risk. Hormonal contraception started immediately after emergency contraception can interact, so confirm the timing with a clinician or pharmacist.
Recognizing true method failure
True failure means the pill was taken within the window and on time, and pregnancy still resulted. This happens in roughly 1–2% of ulipristal users and 2–5% of levonorgestrel users, depending on weight and cycle position. A pregnancy that was already established before the pill was taken (because ovulation had already occurred) looks identical from the outside but reflects the biology of the cycle, not a failure of the medication.
Making the Right Call Right Now and Preventing the Next Emergency
Speed is the single most actionable variable. Count the hours since intercourse, check your body weight, and pick the pill or book the IUD accordingly. The following decision flow captures the essentials:
- Under 24 hours: Either pill works. Levonorgestrel is faster to obtain; ulipristal is stronger if ovulation is imminent.
- 24–72 hours: Levonorgestrel is still an option but declining. Ulipristal is the stronger choice with a prescription.
- 72–120 hours: Ulipristal is the only effective oral option. A copper IUD is the strongest choice overall.
- Over 75 kg or BMI above 30: Lean toward ulipristal or directly to the copper IUD.
- Ovulation likely already occurred: Skip the pills, book a copper IUD within 5 days.
Transitioning to a reliable ongoing method
Emergency contraception is a backup, not a plan. After the immediate crisis is handled, the next step is choosing ongoing contraception that fits your routine. The hormonal IUD, implant, and shot require the least daily attention. The combination pill, patch, and ring offer cycle control but demand consistent use. The copper IUD provides 10 years of hormone-free protection. A clinician visit or telehealth consultation can match you to the right method in under 30 minutes.
What success looks like
Taking the correct option within the right window, confirming with a pregnancy test at three weeks, and choosing a plan for the rest of the cycle turns a panic moment into a managed decision. The copper IUD is the option most people overlook and the one that offers the highest effectiveness, longest duration, and fastest transition from emergency to ongoing care.
That single appointment also sets up a much longer conversation about avoiding the next emergency in the first place.
Key Takeaway
The most important variable in emergency contraception is hours, not the brand on the shelf. Earlier always means more effective, and a copper IUD inserted within 5 days outperforms every pill. Repeated use in one cycle is safe but signals it’s time to switch to ongoing contraception. When in doubt, move fast, pick the option that matches your timing and body weight, and follow up with a pregnancy test at three weeks.
FAQ
How many times can you take the morning-after pill safely?
Clinicians routinely prescribe repeated doses over months or years, and long-term safety data show no cumulative damage to fertility, periods, or hormone balance. It is less effective, more expensive, and causes more side effects than ongoing contraception, so it should not be used as a primary method.
How soon after unprotected sex must you take emergency contraception?
Levonorgestrel works best within 72 hours, while ulipristal acetate remains effective through 120 hours. The sooner either is taken, the higher the effectiveness.
Does taking the morning-after pill multiple times reduce its effectiveness?
No. Each dose works independently based on where you are in your cycle at the time. Repeated use does not build up resistance or make future doses less effective.
Can I take Plan B twice in one week?
Yes, it is safe, but the second dose will be less effective if you are closer to ovulation. A copper IUD is the stronger option if you find yourself needing emergency contraception this often.
What is the difference between Plan B and ella timing?
Plan B (levonorgestrel) is best within 72 hours and stops working after the LH surge begins. Ella (ulipristal acetate) works through 120 hours and can still block ovulation after the LH surge has started.
How does frequent emergency contraceptive use affect fertility?
It does not. No evidence shows that repeated emergency contraception use damages fertility or causes lasting hormonal changes. Normal fertility returns with the next cycle.
