How Effective Is Plan B? Timing, Weight, and Side Effects
Anyone who’s ever done the math after a condom break knows the feeling: you’ve got hours to make a decision, and one big question keeps coming up — how effective is Plan B, really? The answer depends on timing, body weight, and even other medications you might be taking, and the difference between “it probably worked” and “it definitely worked” is narrower than most people think.
Effectiveness within 12 hours: 99% ·
Effectiveness within 24 hours: 95% ·
Effectiveness within 72 hours: 89% ·
Failure rate (typical use): 1-2% ·
Weight limit for optimal efficacy: 155 lbs (70 kg)
Quick snapshot
- Plan B prevents pregnancy by delaying ovulation (FDA)
- Effectiveness decreases over time (WHO)
- Weight can affect efficacy (Planned Parenthood)
- Exact effectiveness for women with BMI >30 (Cadence OTC)
- Whether Plan B works if ovulation has already occurred (MSF Medical Guidelines)
- Exact weight threshold still debated — some studies suggest 155 lbs, others 165 lbs (Northeastern University)
- Take within 72 hours for best results (FDA)
- Some sources say up to 120 hours is possible (Planned Parenthood)
Here is a quick-reference comparison of emergency contraceptive options.
| Method | Time Window | Effectiveness | Weight Limit | Prescription Needed? | Cost |
|---|---|---|---|---|---|
| Plan B (levonorgestrel) | 72 hours | 89% | 155–165 lbs | No | $40–$50 |
| Ella (ulipristal acetate) | 120 hours | 95% | No evidence of limit | Yes | $50–$150 |
| Copper IUD | 5 days | >99% | No limit | Insertion by healthcare provider | $500–$1,000 (but long-term) |
“Plan B works best when you take it within 3 days after unprotected sex.”
— Planned Parenthood
“When taken correctly, Plan B is up to 89% effective. Some clinical trials have reported even better effectiveness (98% in some studies).”
— GoodRx
How likely is it to get pregnant if you took Plan B?
What is the failure rate of Plan B?
- Plan B reduces the risk of pregnancy by about 89% when taken within 72 hours, according to GoodRx (source already cited above).
- Some clinical trials reported up to 98% effectiveness (GoodRx).
- Failure rate is roughly 1-2% with perfect timing (FDA).
Plan B’s headline number — 89% — hides a steep timing curve. For a person who takes it within an hour, the protection is near-complete. For someone who waits until hour 71, the odds shift meaningfully.
How effective is Plan B within 24 hours?
Within the first 24 hours, Plan B is about 95% effective, as the U.S. Food and Drug Administration emphasizes that speed is the single most important factor. The World Health Organization lists levonorgestrel as a first-line emergency contraceptive precisely because it works well when taken promptly.
How effective is Plan B within 3 hours?
Data specific to the 3-hour window is limited, but the principle is clear: earlier equals better. The Médecins Sans Frontières Medical Guidelines recommend taking 1.5 mg levonorgestrel as soon as possible after unprotected intercourse, preferably within the first 72 hours. At 3 hours, effectiveness is near its peak — likely closer to 99% than 89%.
Bottom line: Plan B is a time-sensitive emergency contraceptive that works best within the first day. For the average person who takes it within 24 hours, the failure odds are about 1 in 20. Wait until hour 72, and that risk triples.
The pattern is clear: earlier is dramatically better.
How will I know if Plan B worked?
What are the signs that Plan B prevented pregnancy?
Plan B can cause spotting, breast tenderness, or changes in your menstrual cycle, according to Planned Parenthood. Some people experience nausea or dizziness. But here’s the catch: none of these symptoms reliably mean the pill worked. The only way to confirm is a pregnancy test if your period is delayed by a week or more.
What should I do if I think Plan B failed?
If your period doesn’t arrive within a week of the expected date, take a pregnancy test. GoodRx notes that timing a test too early can give a false negative, so wait until after the missed period. If the test is positive, contact a healthcare provider to discuss options.
What this means: the psychological uncertainty after taking Plan B is real, and it’s normal. The pill either delayed ovulation or it didn’t — and you may not know for two weeks. That’s not a flaw in the drug; it’s the nature of the biological clock it’s trying to reset.
The implication: trust the test, not the symptoms.
At what point is it too late to take Plan B?
Can I take Plan B after 72 hours?
The FDA recommends taking Plan B within 72 hours after unprotected sex. Some sources, including Planned Parenthood, say it can be taken up to 5 days (120 hours) with reduced effectiveness. The MSF Medical Guidelines also note that levonorgestrel may be administered up to 120 hours after unprotected intercourse, but efficacy drops off steeply after 72 hours.
What is the latest time to take Plan B?
At 72 hours, expect roughly 89% effectiveness. At 96 hours, that number is lower — though exact figures are debated. The World Health Organization recommends ulipristal acetate (Ella) or a copper IUD as more effective alternatives for later timing.
A person at hour 73 faces a harder choice: stick with Plan B and accept lower odds, or seek Ella (which requires a prescription in most places) or an IUD insertion. The catch is that the third option requires a clinic visit and may not be available same-day.
The catch: the clock is ticking, and options narrow with every hour.
What cancels a Plan B out?
Does weight affect Plan B effectiveness?
Yes, and the evidence is real. Planned Parenthood says levonorgestrel morning-after pills may not work if a person weighs more than 165 pounds. Northeastern University’s Bouvé College of Health Sciences notes that some studies found reduced effectiveness in people over 165 pounds, but later studies have been conflicting. Currently, the FDA has said there is not enough evidence to add a weight warning to Plan B’s label.
For someone weighing 170 pounds, the official label says nothing about weight limits. Yet the research suggests the drug metabolizes differently. The paradox: a person who reads the label will feel confident, but a person who reads the studies will feel uncertain. Until the FDA re-evaluates, the safest bet for women above 155-165 pounds is Ella or a copper IUD.
The pattern: weight introduces real uncertainty that the label does not address.
Can other medications reduce Plan B’s effectiveness?
Certain enzyme-inducing drugs — including some anti-epileptics, HIV medications, and the antibiotic rifampin — can make Plan B less effective. The FDA notes that these interactions can lower levonorgestrel levels. If you take any of these medications, talk to a pharmacist or doctor about alternatives.
What are the side effects of Plan B?
How long do Plan B side effects last?
Common side effects include nausea, headache, dizziness, breast tenderness, and fatigue, according to Planned Parenthood. Most resolve within 24 hours. If nausea is severe, taking the pill with food may help.
Is Plan B safe to take multiple times?
Plan B is safe for occasional use, but it is not intended as a regular birth control method. Taking it multiple times in one cycle can disrupt your menstrual cycle and cause unpredictable bleeding, but it does not affect long-term fertility, according to GoodRx. For ongoing contraception, consider daily pills, an IUD, or other regular methods.
What this means: Plan B is a backup, not a routine.
Plan B is a highly effective emergency contraceptive when taken promptly, but its success hinges on timing, body weight, and other medications. For most people, taking it within 24 hours gives the best chance of preventing pregnancy.
drugs.com, ncbi.nlm.nih.gov, twentyeighthealth.com, who.int, guttmacher.org
Frequently Asked Questions
Can I take Plan B while breastfeeding?
Yes, Plan B is safe to use while breastfeeding. Small amounts of levonorgestrel may pass into breast milk, but no harmful effects on the infant have been reported. Consult a healthcare provider if you have concerns.
Does Plan B affect future fertility?
No. Plan B does not affect your ability to get pregnant in the future. It works by delaying ovulation and has no lasting impact on fertility.
Can I take Plan B if I’m already pregnant?
Plan B will not end an existing pregnancy. It is designed to prevent pregnancy before it begins. If you are already pregnant, taking Plan B will not harm the fetus.
How often can I take Plan B?
Plan B can be taken as often as needed, but it should not be used as a primary birth control method because it is less effective than regular contraception and may cause menstrual irregularities.
Does Plan B cause miscarriage?
No. Plan B does not cause miscarriage. It works by preventing ovulation or fertilization, not by disrupting an established pregnancy.
What is the difference between Plan B and Ella?
Plan B contains levonorgestrel and is most effective within 72 hours. Ella contains ulipristal acetate and remains effective up to 120 hours. Ella may also be more effective for people with higher body weight. Ella requires a prescription in many places.
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