Emergency contraception (EC) is a critical backup method to prevent pregnancy after unprotected sex or contraceptive failure. While Plan B is a widely known option, many people are unclear about its time constraints. Understanding how late you can have Plan B for it to work and exploring all available options is essential for making an informed and timely decision.
The 72-Hour Window: Plan B's Primary Timeframe
Plan B One-Step, and its generic equivalents containing levonorgestrel, are approved for use within 72 hours (3 days) of unprotected intercourse. It's a single-dose pill that is most effective when taken as soon as possible. The effectiveness is highest when taken within the first 24 hours, with a gradual decrease in effectiveness as time passes. Beyond the 72-hour mark, the effectiveness of levonorgestrel-based pills drops significantly, making them a less reliable option.
How Plan B Works to Prevent Pregnancy
Plan B is not an abortion pill; it works to prevent a pregnancy from ever starting. The active ingredient, levonorgestrel, is a progestin that primarily works by temporarily delaying or stopping ovulation (the release of an egg from the ovary). If ovulation has already occurred, Plan B is no longer effective in preventing pregnancy. It may also thicken cervical mucus to prevent sperm from reaching the egg and alter the uterine lining to inhibit implantation, though delaying ovulation is considered its main mechanism.
Options Beyond the 72-Hour Limit
For situations where more than 72 hours have passed, or for individuals with higher body mass index (BMI), other options are available and may be more effective. Two primary alternatives are the prescription-only pill Ella and the copper intrauterine device (IUD).
Comparison of Emergency Contraception Options
| Feature | Plan B (Levonorgestrel) | Ella (Ulipristal Acetate) | Copper IUD |
|---|---|---|---|
| Timeframe | Most effective within 72 hours (3 days); efficacy decreases with time. | Effective up to 120 hours (5 days); efficacy does not decrease with time. | Effective up to 120 hours (5 days); remains >99% effective throughout the window. |
| Prescription | No prescription required; available over-the-counter. | Prescription required from a healthcare provider. | Requires a doctor's appointment for insertion. |
| Body Weight | Less effective for individuals weighing 155 lbs or more, or with a BMI of 30 or higher. | Less effective for individuals weighing 195 lbs or more. | Not affected by body weight. |
| Mechanism | Delays or prevents ovulation. | Delays ovulation, even after the luteinizing hormone surge begins. | Releases copper ions that are toxic to sperm and eggs, preventing fertilization and implantation. |
| Ongoing Contraception | Does not provide ongoing protection; another method is needed immediately. | Does not provide ongoing protection; can't use hormonal methods for 5 days. | Highly effective long-term contraception (up to 12 years). |
The Copper IUD: The Most Effective Method
The copper IUD is the most effective form of emergency contraception, reducing the risk of pregnancy by over 99% when inserted within 5 days of unprotected sex. Unlike pills, its effectiveness is not affected by body weight. After insertion, it can remain in place to provide highly reliable long-term birth control for up to 12 years, making it an excellent option for those also seeking a future contraceptive method.
Factors That Influence Effectiveness
Several factors can affect how well Plan B works, highlighting why timing is so crucial. Awareness of these factors can help in choosing the most effective emergency contraception for your situation.
The Critical Role of Body Weight
Research suggests that levonorgestrel-based pills, like Plan B, may be less effective for individuals with a higher body mass index (BMI), particularly a BMI of 30 or more, or weighing over 155 pounds. While evidence is sometimes conflicting, it is a significant consideration. The prescription pill Ella also has some potential reduced effectiveness concerns in people weighing over 195 pounds, but it generally performs better than Plan B for those with higher body weights below that threshold. The copper IUD's effectiveness is not impacted by weight.
Drug Interactions with Plan B
Certain medications can interfere with Plan B by speeding up the body's metabolism of the hormones, rendering it less effective. If you are taking any of the following, it is important to consult a healthcare provider for guidance on alternative EC methods:
- Certain seizure medications (e.g., carbamazepine, phenobarbital)
- The antibiotic rifampin
- The herbal supplement St. John's wort
- Some HIV medications
Ovulation Timing and Subsequent Unprotected Sex
Plan B works by delaying ovulation. If you have already ovulated when you take the pill, it will not be effective. Furthermore, Plan B does not offer protection for any unprotected sexual activity that occurs after taking the pill in the same menstrual cycle. This is because the high dose of hormones quickly leaves your system, and ovulation can potentially occur again, leaving you unprotected. Using a barrier method, like condoms, is necessary for any subsequent sexual encounters within the cycle.
What to Do After Taking Plan B
After taking Plan B, you may experience side effects such as nausea, headaches, or breast tenderness, though these are typically mild and temporary. It is common for your next period to be irregular, arriving earlier or later than expected.
- If your period is more than a week late, take a pregnancy test.
- If you vomit within two hours of taking the pill, contact a healthcare provider to see if you need to repeat the dose, as the medication may not have been fully absorbed.
When Emergency Contraception Is Not Right
Emergency contraception is a backup, not a replacement for regular birth control. Repeatedly using EC is less effective and more expensive than using a consistent method like the pill, patch, or IUD. It also does not protect against sexually transmitted infections (STIs), and if you have concerns about STIs, you should see a doctor or visit a sexual health clinic.
Conclusion: Acting Swiftly is Key
The question of how late can you have Plan B for it to work has a clear answer: within 72 hours, but preferably much sooner. Every hour that passes reduces its effectiveness. For those who miss this window, or have other factors that reduce Plan B's efficacy (like higher body weight or drug interactions), other highly effective options like Ella (up to 5 days, by prescription) and the copper IUD (up to 5 days, clinic insertion) are available. Making a quick and informed decision, ideally with advice from a healthcare professional, can make all the difference in preventing an unintended pregnancy. For more comprehensive information on your emergency contraception options, consult a resource like Planned Parenthood.
