Understanding Plan B and Its Primary Function
Plan B One-Step, commonly known as the morning-after pill, is a form of emergency contraception (EC) designed to prevent pregnancy after unprotected intercourse or contraceptive failure [1.2.5]. Its active ingredient is levonorgestrel, a synthetic hormone that primarily works by delaying or preventing ovulation, which is the release of an egg from the ovary [1.2.4, 1.8.1]. It may also thicken cervical mucus, making it more difficult for sperm to reach an egg [1.4.3]. For Plan B to be effective, it must be taken before ovulation occurs; it is not an abortion pill and will not terminate an existing pregnancy [1.2.7, 1.2.4].
The Standard Window of Effectiveness
The recommended and most effective timeframe for taking Plan B is as soon as possible within 72 hours (three days) of unprotected sex [1.2.1, 1.2.5]. The sooner the pill is taken, the higher its efficacy rate.
- Within 24 hours: Studies show Plan B is most effective, with some reporting up to 95% effectiveness in preventing pregnancy [1.3.3, 1.3.2].
- Within 25-48 hours (Day 2): The effectiveness drops to about 85% [1.3.3].
- Within 49-72 hours (Day 3): The effectiveness further decreases to around 58% [1.3.3].
Taking the pill more than 72 hours after intercourse greatly lowers its chances of working [1.2.4].
How Effective is Plan B After 4 Days (96 Hours)?
After the 72-hour mark, the effectiveness of Plan B significantly diminishes. While some sources state that levonorgestrel pills can be taken up to five days (120 hours) after unprotected sex, they are noted to be less effective during this period [1.4.1, 1.4.2]. On the fourth day (96 hours), the pregnancy rate is low, but it increases as time passes toward the fifth day [1.3.4]. Some experts conclude that levonorgestrel is ineffective after 96 hours [1.3.6].
The key reason for this drop in efficacy is timing relative to your menstrual cycle. Plan B works by delaying ovulation. If you have already ovulated, which becomes more likely with each passing day, Plan B will not be effective [1.2.1].
Factors That Can Influence Plan B's Effectiveness
Several factors beyond timing can impact how well Plan B works:
- Body Weight and BMI: Research suggests that Plan B may be less effective for individuals who weigh 165 pounds or more, or have a Body Mass Index (BMI) of 30 or higher [1.2.7, 1.7.2]. However, sources still recommend taking it if it's the only option available, as it may still offer some protection [1.2.7].
- Existing Medications: Certain medications can interfere with levonorgestrel and make it less effective. These include the herbal supplement St. John's wort, some anti-seizure medications (like carbamazepine and phenobarbital), certain HIV drugs, and the antibiotic rifampin [1.2.4, 1.2.7].
- Vomiting: If you vomit within two hours of taking Plan B, your body may not have absorbed the medication, and you should consult a healthcare provider about whether to take another dose [1.2.1, 1.8.5].
- Point in Menstrual Cycle: As mentioned, Plan B is not effective if ovulation has already occurred [1.2.4]. Since it's difficult for most people to know their exact ovulation day, taking it as soon as possible is the best strategy.
More Effective Alternatives After 4 Days
If it has been four days since unprotected intercourse, more effective emergency contraception options are available. The two primary alternatives are the ella pill and a copper IUD.
| Feature | Plan B (Levonorgestrel) | ella (Ulipristal Acetate) | Copper IUD (ParaGard) |
|---|---|---|---|
| Effective Window | Up to 3 days (72 hours) for best results; can be used up to 5 days with reduced efficacy [1.2.1, 1.4.1]. | Up to 5 days (120 hours) [1.2.3, 1.5.6]. | Up to 5 days (120 hours) [1.6.1, 1.6.6]. |
| Effectiveness | 75-89% within 3 days; drops significantly after [1.2.7]. About 58% effective if taken within 120 hours [1.5.2]. | Consistently about 85% effective up to 5 days [1.2.3]. More effective than Plan B, especially after 72 hours [1.5.4]. | Over 99% effective [1.6.1, 1.6.6]. The most effective EC method. |
| Availability | Over-the-counter, no prescription needed [1.2.1]. | Prescription only [1.2.1]. | Requires an appointment for insertion by a healthcare provider [1.6.6]. |
| Weight Impact | May be less effective for individuals over 165 lbs [1.3.7]. | May be less effective for individuals over 195 lbs [1.2.1]. | Effectiveness is not affected by weight [1.6.3]. |
Conclusion: Time is of the Essence
To answer the question, "How effective is Plan B after 4 days?"—its efficacy is significantly reduced and not guaranteed. While it might still offer a small chance of preventing pregnancy, it falls well below its peak effectiveness observed within the first 72 hours. For anyone who finds themselves in need of emergency contraception on day 4 or 5 after unprotected sex, the prescription pill ella or a copper IUD are demonstrably more reliable and recommended options. The copper IUD stands out as the most effective method available and also provides long-term contraception [1.6.1, 1.6.3]. Always consult a healthcare professional to discuss the best and most effective options for your specific situation and health profile.
For more information on emergency contraception, you can visit Planned Parenthood.
