The levonorgestrel pill, commonly known as the “morning-after pill,” is an over-the-counter emergency contraception (EC) designed to prevent pregnancy following unprotected sexual intercourse or contraceptive failure. This medication works by interrupting the biological process that leads to pregnancy. Its success is heavily dependent on how quickly it is used, as it is ineffective once a fertilized egg has implanted in the uterus.
The Critical Time Window for Use
The efficacy of levonorgestrel emergency contraception is directly proportional to the speed of administration. The medication is most effective when taken within 72 hours (three days) of unprotected sexual intercourse. Taking the pill as soon as possible provides the highest chance of preventing a pregnancy.
Studies show that effectiveness can be as high as 95% when the pill is taken within the first 24 hours. The protection rate decreases significantly as time passes, dropping to an estimated 61% when taken between 48 and 72 hours. While some data supports using levonorgestrel EC up to 120 hours (five days) after the event, effectiveness declines sharply after the initial 72-hour window.
Understanding the Mechanism of Action
The primary way the levonorgestrel pill prevents pregnancy is by disrupting the process of ovulation. Ovulation is the release of a mature egg from the ovary, and the high dose of synthetic progestin in the pill works to delay or inhibit this release. By temporarily stopping the egg from being released, the pill ensures that there is no egg available for sperm to fertilize.
If ovulation has already occurred before the pill is taken, the medication is largely ineffective because the window for prevention has closed. The pill may also have secondary effects, such as altering cervical mucus to make it thicker, which impedes the movement of sperm. However, the main mechanism remains pre-ovulatory.
Factors Affecting Efficacy
Several physiological and pharmacological factors can compromise the effectiveness of the levonorgestrel pill. One factor involves body weight, as research suggests the pill may be less effective in individuals with a Body Mass Index (BMI) of 30 or higher, or those who weigh 165 pounds or more. This is thought to be due to altered absorption and metabolism of the hormone, leading to lower drug concentrations in the bloodstream.
Despite these findings, the FDA has not placed an official weight restriction on the levonorgestrel EC label. It is still generally recommended as the first-line option if it is the only one immediately available. Individuals with these weight factors may be advised to consider alternative emergency contraception methods, such as the copper intrauterine device (IUD) or ulipristal acetate, which maintain their efficacy regardless of body weight.
Certain prescription medications and herbal supplements can also significantly decrease the pill’s effectiveness by speeding up the breakdown of levonorgestrel in the body. Medications that induce liver enzymes, such as certain anti-seizure drugs (phenytoin and carbamazepine), the antibiotic rifampin, and St. John’s Wort, reduce the concentration of the hormone. These drug interactions can lower the overall protection provided. Individuals taking these medications should consult a healthcare provider for guidance on alternative emergency contraceptive methods.
What to Expect After Taking Emergency Contraception
After taking the levonorgestrel pill, a person may experience temporary and generally mild physical side effects. If vomiting occurs within two hours of taking the pill, a medical provider should be consulted, as a second dose may be necessary.
Common Side Effects
- Headache
- Dizziness
- Fatigue
- Breast tenderness
- Nausea and abdominal pain
The medication works by altering hormone levels, which will likely affect the timing and characteristics of the next menstrual period. The following period may arrive a few days earlier or later than expected, and it may be lighter or heavier than usual. These changes are a normal result of the high hormone dose.
If the next menstrual period is delayed by more than one week past the expected start date, take a pregnancy test. Emergency contraception is not a substitute for ongoing birth control and does not provide protection for subsequent acts of unprotected sex in the same cycle. It is important to begin using a reliable, long-term contraceptive method immediately after taking the pill.