Understanding Plan B and Its Active Ingredient
Plan B One-Step is a brand name for a single-dose emergency contraceptive pill containing 1.5 mg of levonorgestrel [1.3.2]. Levonorgestrel is a synthetic progestin, a lab-made version of the natural hormone progesterone, which plays a key role in the menstrual cycle and pregnancy [1.3.2, 1.3.3]. The primary way Plan B works is by delaying or preventing ovulation, which is the release of an egg from the ovary [1.7.4]. It may also thicken cervical mucus, making it harder for sperm to reach an egg [1.3.2]. It is important to note that Plan B is not an abortion pill and will not terminate an existing pregnancy [1.3.2]. It is available over-the-counter without a prescription [1.9.1].
Pharmacokinetics: How Your Body Processes Plan B
The key to understanding how long Plan B stays in your system lies in its pharmacokinetics, specifically its half-life. The half-life of a drug is the time it takes for the concentration of the drug in the body to be reduced by half [1.2.5]. For the levonorgestrel in Plan B, the mean elimination half-life is approximately 24 to 27.5 hours [1.2.1, 1.3.2].
This means that after about a day, half of the initial dose has been metabolized and eliminated. It generally takes about five half-lives for a drug to be almost completely cleared from the system [1.2.5]. Therefore, small amounts of Plan B may remain in your body for up to 5 or 6 days after taking the pill [1.3.2, 1.3.4]. However, its peak effectiveness is within the first 24-72 hours, and its presence for several days does not mean it continues to provide contraceptive protection [1.3.2].
The body eliminates levonorgestrel and its metabolites primarily through urine (about 45%) and feces (about 32%) [1.2.1].
Factors Influencing How Long Plan B Stays in Your System
Several individual factors can influence the metabolism and clearance rate of levonorgestrel, meaning the exact duration it remains in the body can vary.
Body Weight and BMI
Body weight and Body Mass Index (BMI) are significant factors. Studies suggest that levonorgestrel-based emergency contraception, like Plan B, may be less effective in individuals with a higher BMI [1.8.2]. For instance, some research indicates reduced effectiveness for those weighing over 165 pounds (75 kg) or with a BMI of 30 or higher [1.8.2, 1.8.5]. This may be due to factors like a larger blood volume diluting the medication or the hormone being stored in fat cells, which lowers the active concentration in the bloodstream [1.8.2]. One study found that obese women had significantly lower maximum concentrations of levonorgestrel compared to women with a normal BMI, though a double dose appeared to correct this difference [1.4.5].
Liver Function and Other Medications
Levonorgestrel is metabolized in the liver, primarily by the CYP3A4 enzyme [1.2.1]. Therefore, individual differences in liver function can affect how quickly the drug is processed. Furthermore, other medications can interfere with its metabolism [1.2.2]. Certain drugs, including some anticonvulsants, antibiotics, and the herbal supplement St. John's Wort, can speed up the breakdown of levonorgestrel, potentially reducing its effectiveness [1.2.2].
Comparison of Emergency Contraception Options
It's useful to compare Plan B with other available emergency contraception (EC) methods.
| Feature | Plan B (Levonorgestrel) | ella (Ulipristal Acetate) | Copper IUD (ParaGard) |
|---|---|---|---|
| Active Ingredient | 1.5 mg Levonorgestrel [1.3.2] | 30 mg Ulipristal Acetate [1.9.3] | Copper [1.5.3] |
| Effective Window | Up to 3 days (72 hours), most effective sooner [1.5.3] | Up to 5 days (120 hours) [1.5.3] | Up to 5 days (120 hours) [1.5.3] |
| Effectiveness | 75-89% [1.5.1] | More effective than Plan B, especially closer to day 5 [1.5.6] | Over 99% effective [1.5.1] |
| Effect of Weight/BMI | May be less effective with BMI >25-30 [1.8.1, 1.8.2] | May be less effective with BMI >35 [1.9.3] | Not affected by weight/BMI [1.8.1] |
| Availability | Over-the-counter (no prescription) [1.5.2] | Prescription only [1.5.2] | Requires provider insertion [1.5.1] |
Side Effects and Menstrual Cycle Impact
After taking Plan B, some people may experience side effects, which are generally mild and temporary, often lasting only a day or two [1.6.2, 1.6.5]. Common side effects include nausea, headache, abdominal pain, fatigue, breast tenderness, dizziness, and changes in menstrual bleeding [1.6.2].
One of the most common effects is a change to the next menstrual period. Your period may come earlier or later than expected (often by about a week), and the flow may be heavier, lighter, or more spotty [1.7.2, 1.7.3]. This is due to the high dose of hormone temporarily disrupting your cycle [1.7.1]. If your period is more than a week late, it is recommended to take a pregnancy test [1.7.2].
Conclusion
While the active hormone in Plan B, levonorgestrel, can be detected in the body for about five to six days, its effective window for preventing pregnancy is much shorter, ideally within 72 hours of unprotected intercourse [1.3.2]. The drug's half-life of about 24-27 hours means its concentration drops significantly after the first day [1.2.1]. Factors like body weight, BMI, and interactions with other medications can influence how the body processes it [1.4.5, 1.2.2]. While generally safe, it can cause temporary side effects and changes to the menstrual cycle [1.6.2, 1.7.3]. For those concerned about effectiveness due to weight or the timing of unprotected sex, other options like ella or a copper IUD may be more suitable [1.5.4, 1.8.1].
For more information, you can visit the FDA's page on Birth Control.
