Emergency contraception, commonly known as Plan B, offers a way to prevent pregnancy after unprotected sexual intercourse or when a primary birth control method fails. This medication is designed for situations such as a broken condom, missed birth control pills, or sexual assault. Plan B contains levonorgestrel, a synthetic hormone also found in lower doses in many regular birth control pills. Its purpose is to intervene before pregnancy is established, providing a time-sensitive option to reduce the chance of conception.
The Critical Time Window
Plan B’s effectiveness depends on how quickly it is taken after unprotected sex. For levonorgestrel-based emergency contraception, the primary effective window is within 72 hours, or three days, of the unprotected act. Its effectiveness decreases as more time passes, making prompt use beneficial. While most effective within 72 hours, some studies indicate it can still offer moderate effectiveness up to 120 hours, or five days, after unprotected intercourse.
This timeframe is important because Plan B primarily works by delaying or preventing ovulation (the release of an egg from the ovary). If ovulation has already occurred, the medication will not prevent pregnancy. Another form of emergency contraception, ulipristal acetate (often sold as ellaOne), can be effective for a longer period, up to 120 hours or five days, after unprotected sex. For Plan B, taking it as soon as possible significantly increases its success.
How Emergency Contraception Works
Plan B contains a higher dose of levonorgestrel, a progestin hormone. It primarily prevents pregnancy by interfering with ovulation. It suppresses the luteinizing hormone (LH) surge, necessary for an egg to be released from the ovary. By delaying or inhibiting ovulation, Plan B ensures no egg is available for sperm to fertilize, preventing conception.
It may also thicken cervical mucus, which can hinder sperm. However, its main action is on ovulation. Plan B is not an abortion pill; it does not terminate an existing pregnancy. If implantation has already occurred, Plan B will not be effective or disrupt the pregnancy.
Factors Influencing Effectiveness
Several factors can influence how well Plan B works. As previously mentioned, taking the pill sooner rather than later is directly linked to its efficacy. The longer the delay, the less likely it is to prevent pregnancy.
Body weight can also affect the effectiveness of levonorgestrel-based emergency contraception. Research suggests that Plan B may be less effective for individuals with higher body weights, particularly those weighing over 155 pounds. While Plan B has no specific weight limit, alternative emergency contraception methods, such as the copper intrauterine device (IUD) or ulipristal acetate, may be considered for individuals in higher weight ranges.
Certain medications can also reduce its effectiveness. These include:
- Specific anti-seizure medications (e.g., carbamazepine, phenytoin, phenobarbital)
- Rifampin (an antibiotic)
- The herbal supplement St. John’s wort
- Some HIV medications (e.g., efavirenz)
Consult a healthcare provider or pharmacist if you are taking other medications, as they can assess potential interactions. Plan B only prevents pregnancy from the unprotected sexual act that occurred before taking the pill; it offers no ongoing protection for subsequent acts of unprotected sex.
After Taking Plan B
After taking Plan B, individuals may experience common, mild side effects. These can include nausea, vomiting, fatigue, headache, dizziness, and breast tenderness. If vomiting occurs within two hours, the pill may not have been fully absorbed; seek medical advice for another dose.
The next menstrual period might differ from usual. It could arrive earlier, later, be lighter or heavier, or involve some spotting. These changes are common and usually resolve by the next cycle. If the next period is delayed by over a week or is unusually light, taking a pregnancy test is advisable.
Also take a pregnancy test if experiencing any pregnancy symptoms. Seek medical advice if severe abdominal pain or very heavy bleeding occurs, or if there are concerns about the pill’s effectiveness or a potential existing pregnancy. Plan B is for emergency situations only and is not a substitute for regular, ongoing birth control methods.