The Primary Mechanism: Delaying Ovulation
The 72h pill is an emergency contraceptive that uses a high dose of a synthetic hormone called levonorgestrel. This powerful dose affects the hormonal processes that govern a person's menstrual cycle. Its primary mechanism of action is to prevent or delay ovulation, which is the release of an egg from the ovary.
To understand this process, it is important to know that a surge in Luteinizing Hormone (LH) in the body is what triggers ovulation. The high dose of levonorgestrel in the emergency pill works by suppressing this LH surge. If ovulation is delayed or prevented, any sperm present from unprotected intercourse will not find an egg to fertilize, and pregnancy cannot occur. The pill is most effective when taken before the LH surge has started, highlighting why earlier use is more successful.
Secondary Modes of Action
Beyond its primary effect on ovulation, levonorgestrel has other mechanisms that can help prevent pregnancy, though these are considered secondary to ovulation delay:
- Thickening cervical mucus: The hormone can increase the viscosity of cervical mucus, making it more difficult for sperm to travel through the cervix and reach an egg.
- Altering the uterine lining: In some cases, the medication may cause changes to the lining of the uterus, potentially making it less receptive to implantation. However, recent and abundant evidence suggests this effect is not its primary mechanism for emergency use and that it does not affect implantation after ovulation has already occurred.
Effectiveness, Timing, and Limiting Factors
The effectiveness of the 72h pill is highly dependent on when it is taken after unprotected sex. The sooner it is taken, the better it works. While it is effective for up to 72 hours (three days), its efficacy starts to decline after the first 24 hours.
Effectiveness over time (Levonorgestrel):
- 0-24 hours: Up to 94% effective when taken correctly within this window.
- 25-72 hours: The effectiveness decreases as the hours pass.
Additional factors that can impact efficacy include:
- Stage of menstrual cycle: The pill is not effective if ovulation has already occurred.
- Body weight: Some evidence suggests that levonorgestrel-based pills may be less effective for individuals with a higher body mass index (BMI) or weighing over 165 pounds. For these individuals, a 120h pill or a copper IUD may be recommended.
Comparing Different Emergency Contraceptives
Not all emergency contraceptive pills are the same. A key difference lies in the active ingredient and the window of effectiveness. Here is a comparison of the different types available:
| Feature | 72h Pill (Levonorgestrel) | 120h Pill (Ulipristal Acetate) | Copper IUD |
|---|---|---|---|
| Active Ingredient | Levonorgestrel | Ulipristal Acetate | Copper |
| Timeframe | Up to 72 hours (3 days) | Up to 120 hours (5 days) | Up to 120 hours (5 days) |
| Availability | Over-the-counter (OTC) at pharmacies and drugstores. | Prescription only. | Requires insertion by a healthcare provider. |
| Effectiveness | Highest within 24 hours, decreases over time. | Highly effective throughout the 120-hour window. | Over 99% effective, long-term option. |
| Main Mechanism | Delays/inhibits ovulation. | Blocks progesterone, delays ovulation even closer to the LH surge. | Creates a toxic environment for sperm and eggs; also prevents implantation. |
| Body Weight Effect | Can be less effective for people over 165 lbs. | Efficacy is not reduced by body weight. | Efficacy is not affected by body weight. |
Common Side Effects and What to Expect
Due to the high hormonal dose, taking the 72h pill can cause temporary side effects. These typically subside within a day or two.
Possible side effects include:
- Nausea and/or vomiting
- Headache
- Dizziness
- Fatigue
- Breast tenderness
- Abdominal pain or cramps
- Menstrual irregularities, such as an earlier or later period, or changes in bleeding.
If vomiting occurs within two hours of taking the pill, it is important to contact a healthcare professional, as another dose may be needed.
Clarifying Common Misconceptions
It is crucial to understand that the 72h pill is an emergency contraceptive, not an abortion pill. The two are fundamentally different medications with different mechanisms.
- Emergency contraceptive pills (like levonorgestrel) work to prevent pregnancy from occurring in the first place, primarily by interfering with ovulation.
- Abortion pills (like mifepristone) are used to terminate an existing pregnancy. They work by blocking the hormone progesterone, which is necessary for a pregnancy to continue.
The 72h pill will not harm an existing pregnancy and does not terminate a fertilized egg after it has implanted in the uterus.
Conclusion
Understanding how the 72h pill works is essential for making informed decisions about sexual health. As a backup method, it provides a safe and effective way to prevent unintended pregnancy after unprotected sex. The key to maximizing its effectiveness is to take it as soon as possible after the event. While it offers a valuable safety net, it is not intended for regular use and is less effective than routine birth control methods. For ongoing contraception, discussing long-term options like daily pills, implants, or IUDs with a healthcare provider is highly recommended. For more information, you can also consult authoritative sources such as Planned Parenthood or the Mayo Clinic.
