Understanding Plan B and the 72-Hour Window
Plan B One-Step and its generic versions are a form of emergency contraception (EC), often called the "morning-after pill." The primary active ingredient is levonorgestrel, a synthetic hormone that works mainly by preventing or delaying ovulation, which is the release of an egg from the ovary [1.3.4, 1.3.5]. It may also thicken cervical mucus, making it harder for sperm to reach an egg [1.3.5]. It is crucial to understand that Plan B is not an abortion pill; it does not terminate an existing pregnancy and will not harm an established pregnancy [1.4.4, 1.2.6].
The question many people have is: Is Plan B useless after 72 hours? The simple answer is no, it is not entirely useless, but its effectiveness drops significantly. Emergency contraceptive pills are most effective the sooner they are taken [1.2.3]. Studies show that levonorgestrel pills are most effective when taken within 24 hours of unprotected sex, with an efficacy rate as high as 95% [1.2.2]. This rate drops to about 85% if taken within 25 to 48 hours, and to 58% if taken between 49 and 72 hours [1.2.2, 1.2.3]. While some data suggests it may have moderate effectiveness up to five days after intercourse, the chances of pregnancy increase with each day that passes [1.2.4].
How Efficacy Changes Over Time
The effectiveness of levonorgestrel-based EC is highly time-dependent. Here's a general breakdown based on available research:
- Within 24 Hours: This is the most effective window, preventing an estimated 95% of expected pregnancies [1.2.2].
- 25–48 Hours: The effectiveness drops but remains relatively high, preventing about 85% of pregnancies [1.2.2].
- 49–72 Hours: The efficacy decreases to around 58-61% [1.2.2, 1.2.5].
- After 72 Hours (3 days): The effectiveness is greatly lowered [1.2.7]. While it might still offer some protection up to 120 hours (5 days), other options are considered more reliable during this later timeframe [1.2.4, 1.4.4].
Factors That Can Influence Plan B's Effectiveness
Beyond timing, other factors can impact how well Plan B works.
Body Weight and BMI
Research suggests that levonorgestrel pills like Plan B may be less effective for individuals with a higher body mass index (BMI) [1.2.6]. Some studies have indicated that the pill may be less effective in people weighing over 155-165 pounds or with a BMI of 25-30 or higher [1.4.4, 1.6.2, 1.6.3]. This might be because the standard hormone dose is less effective in individuals with higher body weight and blood volume [1.6.4]. However, the FDA has not recommended a labeling change, and guidance often states that individuals should not be restricted from using it based on weight [1.2.6, 1.6.2].
Timing Within Your Menstrual Cycle
Plan B's primary mechanism is delaying ovulation. Therefore, if you have already ovulated, the pill is not likely to be effective [1.2.7, 1.4.2]. Since it can be difficult to know the exact timing of ovulation, it is recommended to take EC as soon as possible regardless of where you are in your cycle [1.4.4].
Other Medications
Certain medications and herbal supplements can interfere with Plan B's effectiveness by affecting how it's metabolized in the body. These include the antibiotic rifampin, some medications for epilepsy or HIV, and the herbal remedy St. John's Wort [1.6.2].
Alternatives to Plan B, Especially After 72 Hours
If it has been more than 72 hours since unprotected sex, or if you have concerns about weight-related efficacy, there are more effective alternatives.
| Feature | Plan B (Levonorgestrel) | ella (Ulipristal Acetate) | Copper IUD (ParaGard) |
|---|---|---|---|
| Time Window | Most effective within 72 hours (3 days) [1.2.3] | Effective up to 120 hours (5 days) [1.4.4] | Effective up to 120 hours (5 days) [1.5.5] |
| Effectiveness | 75-89% within 3 days, but declines over time [1.4.3] | ~85% effective up to 5 days [1.4.7] | Over 99.9% effective [1.5.5] |
| Prescription? | No, available over-the-counter [1.4.4] | Yes, prescription required [1.4.4] | No, but must be inserted by a clinician [1.5.5] |
| Weight Impact | May be less effective if over 165 lbs or BMI >25 [1.4.4, 1.6.3] | May be less effective if over 195 lbs or BMI >35 [1.4.4, 1.6.3] | No weight limitations [1.5.2, 1.6.3] |
| How It Works | Primarily delays or inhibits ovulation [1.3.4] | Delays ovulation, even closer to its onset [1.4.4] | Prevents fertilization by being toxic to sperm [1.5.2] |
- ella® (ulipristal acetate): This is another emergency contraceptive pill that is more effective than Plan B, especially between 72 and 120 hours after unprotected sex. It requires a prescription and maintains its effectiveness throughout the 5-day window [1.4.3, 1.4.4]. It works by delaying ovulation and may be more effective than Plan B for individuals with a higher BMI [1.4.3].
- Copper IUD (ParaGard): The copper IUD is the most effective form of emergency contraception, with an efficacy rate of over 99.9% [1.5.5]. A healthcare provider must insert it within five days of unprotected sex. It is hormone-free and has no weight limitations [1.5.2]. A major benefit is that it can remain in place and serve as a highly effective long-term birth control method for up to 10-12 years [1.5.2, 1.5.5].
Common Side Effects
Side effects of levonorgestrel pills like Plan B are usually mild and short-lived. They can include changes to your next period (it might be earlier, later, heavier, or lighter), nausea, lower abdominal pain or cramps, fatigue, headache, and dizziness [1.7.2, 1.7.1]. If you vomit within two hours of taking the pill, you should contact a healthcare provider, as you may need to take another dose [1.4.6].
Conclusion: Act Quickly and Know Your Options
To answer the primary question: no, Plan B is not completely useless after 72 hours, but its effectiveness drops considerably. The most critical factor for Plan B is to take it as soon as possible after unprotected sex. If more than three days have passed, or if you have concerns about factors like body weight, more effective options like the prescription pill ella or a copper IUD are available. The copper IUD is the most effective emergency contraception method and also provides long-term protection [1.5.5]. Speaking with a healthcare provider can help you decide the best and most effective course of action for your situation.
For more information on emergency contraception, you can visit Planned Parenthood.
