Plan B, also known as the morning-after pill, is an emergency contraceptive containing levonorgestrel. It helps reduce the chance of pregnancy after unprotected sex or when a primary birth control method fails. This medication serves as a backup, not a routine form of contraception.
The Critical Time Window
Plan B is most effective when taken as soon as possible after unprotected sex. It is recommended for use within 72 hours, or three days, of the event. Taking it within this initial window significantly increases its ability to prevent pregnancy, with effectiveness as high as 89%.
While the first 72 hours offer the highest efficacy, Plan B may still be effective up to 120 hours, or five days, after unprotected sex. However, its effectiveness decreases considerably with each passing day. The sooner the pill is taken, particularly within the first 24 hours, the better its chances of working.
How Plan B Works
Plan B primarily functions by preventing or delaying ovulation, the release of an egg from the ovary. Without an egg available for fertilization, pregnancy cannot occur. The levonorgestrel in Plan B temporarily suppresses the hormonal surge that triggers ovulation.
This medication may also thicken cervical mucus, creating a barrier that makes it more difficult for sperm to reach an egg. It can also cause changes to the uterine lining, though this is not its main mechanism. Plan B is not an abortion pill; it does not end an existing pregnancy.
Factors Influencing Effectiveness
Beyond timing, other factors can influence Plan B’s effectiveness. Body weight is a consideration; research suggests levonorgestrel-based emergency contraception may be less effective for individuals weighing over 155 pounds or with a Body Mass Index (BMI) above 25. Some studies indicate a more significant decrease in efficacy at weights above 175-176 pounds. While there is no official weight limit on the product label, individuals in higher weight categories might discuss alternative emergency contraception with a healthcare provider.
Certain medications can also reduce Plan B’s efficacy by affecting how the body processes levonorgestrel. These include anti-seizure medications like topiramate, carbamazepine, phenytoin, and phenobarbital. The herbal supplement St. John’s Wort, along with drugs such as rifampin and efavirenz, can also interfere with its action. Inform a pharmacist or healthcare provider about all current medications and supplements when seeking emergency contraception.
Other Emergency Contraception Options and Follow-Up
If Plan B is not suitable or the 72-hour window has passed, other emergency contraception options are available. Ulipristal acetate (Ella) is an oral emergency contraceptive that can be taken up to 120 hours (five days) after unprotected sex. Ella requires a prescription and may be more effective than levonorgestrel-based pills for individuals with higher body weights.
The copper intrauterine device (IUD) is another highly effective emergency contraception method. It can be inserted by a healthcare provider up to 120 hours (five days) after unprotected sex. The copper IUD is considered the most effective form of emergency contraception and can then serve as a long-term birth control method.
After taking Plan B, the next menstrual period may differ from usual (earlier, later, heavier, or lighter). If a period is more than a week late, or if pregnancy symptoms develop, take a pregnancy test. A pregnancy test should be taken at least three weeks after unprotected sex for accurate results. Seek medical advice if severe abdominal pain occurs, as this could indicate a rare but serious condition like an ectopic pregnancy.