Plan B, also known as levonorgestrel emergency contraception, serves as a backup method to prevent pregnancy after unprotected sexual intercourse or when a regular contraceptive method fails. This medication is specifically designed for emergency situations, offering a time-sensitive option to reduce the chance of an unplanned pregnancy. It is not intended for routine use as a primary birth control method.
How Plan B Works
Plan B primarily works by delaying or preventing ovulation, which is the release of an egg from the ovary. It contains a higher dose of levonorgestrel, a synthetic hormone similar to progesterone, which is also found in lower doses in some daily birth control pills. By interfering with ovulation, Plan B aims to ensure no egg is available for fertilization. This emergency contraceptive does not terminate an existing pregnancy. If ovulation has already occurred before the pill is taken, Plan B will not be effective.
The Critical Timeframe for Efficacy
The effectiveness of Plan B is highly dependent on how quickly it is taken after unprotected sex. It is approved for use within 72 hours (three days), with its efficacy decreasing significantly the longer the delay. Taking the pill as soon as possible offers the greatest chance of preventing pregnancy. When taken within 24 hours, Plan B can be around 94% to 95% effective. This effectiveness rate can drop to approximately 58% to 89% when taken within 72 hours. While some sources suggest it may offer moderate effectiveness up to 120 hours (five days), this is with a considerably reduced success rate.
Factors Affecting Efficacy Over Time
The timing of intake remains a primary factor influencing Plan B’s effectiveness, as it works best the sooner it is administered following unprotected intercourse. Its ability to prevent pregnancy declines notably after the initial 72-hour window.
Body weight can also play a role, with studies indicating that Plan B may be less effective in individuals with a higher body mass index (BMI), particularly those with a BMI of 25-29 or above 30. This reduced efficacy is thought to be due to how the drug is processed in the body at higher weights.
Certain medications can interfere with Plan B’s action, making it less effective. These include some anti-seizure drugs, such as carbamazepine, phenytoin, and topiramate, as well as rifampin and the herbal supplement St. John’s Wort. Consulting a healthcare provider about all medications is advisable to identify potential interactions.
Plan B is not designed for regular use. While safe to take multiple times if needed, it is less effective than routine contraceptive options and can lead to more frequent side effects like menstrual irregularities. It provides no ongoing protection against pregnancy if unprotected sex occurs again after taking the pill.
Important Considerations and Next Steps
Plan B functions as a pregnancy preventative and does not end an existing pregnancy. It also does not offer any protection against sexually transmitted infections (STIs). For STI prevention, barrier methods like condoms are necessary.
If a period is delayed by more than a week after taking Plan B, it is recommended to take a pregnancy test. Any concerns about effectiveness, potential side effects, or questions about other options should prompt a consultation with a healthcare provider.
Discussing ongoing birth control methods with a healthcare professional is advisable for future pregnancy prevention. Regular contraception offers more consistent and reliable protection compared to emergency options.