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Does Plan B Work? Side Effects & Effectiveness
By: Healthtime Editorial
Fact checked by: QA Team
Updated on: February 3, 2026
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6 min
In this article
- Key Differences at a Glance: Plan B vs. Other Options
- How Ovulation Timing Affects Success
- 3 Other Factors That Influence Effectiveness
- How to Know If Plan B Worked
- What to Do If Plan B Fails
- FAQ
Does Plan B always work? The short answer is no, it is not 100% effective.
While it significantly reduces the risk of pregnancy, its success depends heavily on timing and where you are in your menstrual cycle.
This guide is for anyone who has taken emergency contraception or is considering it and wants to understand the variables at play.
I will cover the key effectiveness rates, how ovulation impacts results, and what signs to look for in the days following.
Key Differences at a Glance: Plan B vs. Other Options
While Plan B is the most well-known emergency contraceptive, it is not the only option available.
Understanding how it compares to other methods can help you choose the most effective solution for your specific timeline and body type.
Digital Tools for Cycle Awareness
Understanding your body's natural rhythm is empowering for reproductive health and decision-making.
Tools like Cycle Diet help individuals track their unique hormonal patterns and fertile windows through nutrition and lifestyle.
Having this awareness beforehand can make navigating emergency situations less stressful.
Comparison of Emergency Contraceptive Methods
Choosing the right method depends on how much time has passed since unprotected sex.
Here is a quick breakdown of how the most common options stack up against each other.
| Method | Window | Efficacy | Weight Limit | Script Needed |
|---|---|---|---|---|
Plan B (Levonorgestrel) | Up to 72 hours | Use dependent | Less effective >165 lbs | No (OTC) |
Ella (Ulipristal) | Up to 5 days | Consistent 5 days | Less effective >195 lbs | Yes |
Copper IUD | Up to 5 days | Over 99% | No impact | Requires procedure |
When to Choose an Alternative to Plan B
There are specific scenarios where Levonorgestrel might not be your best defense.
If it has been 4 or 5 days since unprotected intercourse, Ella or a Copper IUD are significantly more effective choices.
Additionally, research suggests that for individuals weighing over 75 kg (165 lbs) or with a BMI over 25, the standard dose may be less potent.
In these cases, speaking to a pharmacist about alternatives can greatly increase your chances of preventing pregnancy.
Beyond choosing the right method, understanding the biological mechanism is key to knowing if it will work for you.
How Ovulation Timing Affects Success
The most significant limitation of Plan B is that it works by delaying ovulation, not by disrupting an existing pregnancy.
If your body has already released an egg, the medication may not be effective.
How Plan B Pauses the Ovaries
Levonorgestrel works by introducing a high dose of synthetic progesterone into your system.
This surge signals the ovaries to hold off on releasing an egg, effectively hitting the 'pause' button.
However, this mechanism relies entirely on timing.
It must be taken before the luteinizing hormone (LH) surge triggers the release of the egg.
Effectiveness After Ovulation Has Begun
If ovulation has already occurred within the last 24 hours, an egg is present in the fallopian tube.
In this specific window, fertilization can still happen even if you take the pill immediately.
This is why the pill is not considered a guaranteed fix for everyone.
It cannot retrieve an egg that has already been released.
Estimating Your Ovulation Risk
To guess if you are at risk, check your cycle dates; ovulation typically happens around day 14 of a 28-day cycle.
You might also notice physical signs like clear, stretchy cervical mucus which indicates high fertility.
1. Check the calendar — Count days from the first day of your last period.
2. Observe symptoms — Look for changes in discharge or mild pelvic pain.
3. Assess risk — If you are mid-cycle, the risk of failure is higher.
Many people use period tracker apps, but remember these are estimates and not guarantees.
Clarifying the Implantation Myth
It is important to state clearly that Plan B does not affect the implantation of a fertilized egg.
Medical consensus and FDA labeling confirm it is not an abortion pill and cannot end an established pregnancy.
If a fertilized egg has already implanted in the uterus, the medication will simply have no effect.
While ovulation status is the biggest factor, other external factors like weight and timing also play a crucial role.
3 Other Factors That Influence Effectiveness
Apart from your cycle phase, several physiological and external factors can impact how well the morning-after pill is absorbed and utilized by your body.
Being aware of these can help you manage expectations and choose the best course of action.
Body Weight and BMI Considerations
Clinical data suggests that efficacy may decrease for individuals with a BMI over 25 or a body weight exceeding 165 lbs (75 kg).
This does not mean it won't work, but the risk of failure is statistically higher.
Generally, taking a double dose is not recommended without doctor supervision due to side effects.
In these instances, a Copper IUD or Ella is often suggested as a more reliable alternative.
The 72-Hour Window and Diminishing Returns
Time is of the essence when it comes to emergency contraception.
The pill is most effective (up to 95%) when taken within the first 24 hours after unprotected sex.
However, this effectiveness drops to approximately 61% between 48 and 72 hours.
The outcome is clear: the sooner you take it, the better it works.
Interactions With Daily Medications
Certain chronic medications can speed up how your liver processes hormones, reducing the pill's staying power.
Common culprits include some anticonvulsants, St. John's Wort, and certain HIV treatments.
If you take any daily medication, a quick check with a pharmacist is essential to ensure you are covered.
Vomiting After Taking the Dose
Nausea is a common side effect, but vomiting can be a problem for absorption.
If you vomit within 2 hours of taking the pill, your body likely hasn't absorbed enough of the medication.
In this case, you should contact a healthcare provider immediately to determine if a replacement dose is needed.
After taking the pill, you might be anxious to know if it worked; here is what to look for.
How to Know If Plan B Worked
There is no immediate physical sign or sensation that confirms the pill has successfully prevented pregnancy.
The only definitive proof is the arrival of your next menstrual period.
Changes to Your Next Period
Plan B can cause your next period to arrive earlier or later than usual.
You might also notice the flow is lighter or heavier than what is normal for you.
A late period does not automatically mean pregnancy, but it is a signal to pay attention to.
If your period is more than a week late, it is time to take a test.
Differentiating Side Effects from Pregnancy Symptoms
The high dose of hormones often causes side effects that mimic early pregnancy.
Symptoms like nausea, breast tenderness, and fatigue are common reactions to the medication.
Try not to panic if you feel these symptoms shortly after dosing, as they are likely just side effects.
When to Take a Pregnancy Test
Testing too early can result in false negatives, providing a false sense of security.
It is best to wait until your period is at least one week late to get an accurate result.
Alternatively, you can test at least 3 weeks after the unprotected sex occurred.
If you discover that the medication did not work, it is important to know your options moving forward.
What to Do If Plan B Fails
If your period does not arrive or you receive a positive pregnancy test, it indicates that the emergency contraception was not successful.
In this situation, prompt medical guidance is essential to ensure your health and safety.
Confirming the Pregnancy
Home tests are accurate, but a blood test at a clinic can provide definitive confirmation.
Schedule an appointment with your primary care doctor or a reproductive health clinic as soon as possible.
Rest assured that taking Plan B does not typically harm a developing fetus if the pregnancy continues.
Exploring Your Options
Once pregnancy is confirmed, you should speak with a healthcare provider to discuss your reproductive choices.
They can provide counseling and medical support tailored to your decision.
Remember that early medical consultation gives you the most time and options.
FAQ
Can you get pregnant if you take Plan B?
Yes, it is possible to get pregnant even after taking the pill.
If you have already ovulated or if the medication was taken too late, it may not prevent pregnancy.
Does Plan B work during ovulation?
If you are currently ovulating or have just released an egg, Plan B is unlikely to be effective.
Its primary mechanism is to stop the egg from being released in the first place.
How effective is Plan B after 3 days?
After 3 days (72 hours), the effectiveness of Plan B drops significantly.
Some studies suggest it may still offer some protection up to 120 hours, but methods like Ella or a Copper IUD are much better choices at this stage.
Does bleeding mean Plan B worked?
Spotting or bleeding a few days after taking the pill is a common side effect, not proof of success.
The only way to know for sure is when your full menstrual period arrives at its expected time.
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