Plan B (levonorgestrel emergency contraception) is a backup method to prevent pregnancy after unprotected sex or contraceptive failure. It reduces the chance of pregnancy when taken within a specific timeframe. Understanding its mechanism and how factors like body weight can influence its effectiveness is important for proper use.
How Plan B Works
Plan B primarily functions by temporarily delaying or preventing ovulation, the release of an egg from the ovary. This ensures no egg is available for fertilization. Plan B works before a pregnancy is established and will not terminate an existing pregnancy. It also does not protect against sexually transmitted infections.
The active ingredient, levonorgestrel, is a synthetic hormone mimicking progesterone. This hormone can also thicken cervical mucus, impeding sperm movement. Plan B is most effective when taken as soon as possible after unprotected sex, generally within 72 hours.
Weight and Plan B Effectiveness
Research indicates Plan B (levonorgestrel) may be less effective in individuals above certain body weights or Body Mass Index (BMI) levels. Studies suggest reduced effectiveness for those weighing around 155-165 pounds (70-75 kg) or with a BMI of 25-30 kg/m² or higher. This potential reduction in effectiveness is thought to occur because people with a higher body mass may experience lower concentrations of the drug in their bloodstream.
The exact reasons are not fully understood, but possibilities include differences in drug absorption, metabolism, or distribution. While some studies show an increased risk of pregnancy for individuals in higher weight categories, other reviews suggest pregnancy rates remain low across different BMI and weight groups after taking levonorgestrel.
Plan B is a single-dose medication; taking more does not increase its effectiveness. Despite these findings, the Food and Drug Administration (FDA) has not changed its official recommendations regarding weight limits for Plan B.
Exploring Other Emergency Contraception
For individuals concerned about Plan B’s effectiveness due to weight, or seeking highly effective alternatives, other emergency contraception methods are available. One option is ella (ulipristal acetate), a prescription-only oral medication. Ella works by delaying or inhibiting ovulation and can be effective even closer to the time of ovulation than Plan B. Its effectiveness is generally less affected by weight, though some data suggest a potential decrease for individuals over 195 pounds.
The copper intrauterine device (IUD) is another highly effective method. This non-hormonal device can be inserted by a healthcare provider up to five days after unprotected sex. The copper IUD works by creating an environment toxic to sperm, preventing fertilization and implantation. Its effectiveness is not affected by body weight, making it suitable for all individuals.
Essential Steps After Taking Plan B
After taking Plan B or any emergency contraception, monitor for your next menstrual period. Your period might be earlier, later, or have a different flow. If your period is significantly delayed by more than a week, or if you have concerns, take a pregnancy test.
Most home pregnancy tests provide accurate results if taken approximately three weeks after the unprotected sexual encounter or a missed period. Consult a healthcare provider to discuss concerns, explore ongoing birth control options, or if emergency contraception was used due to weight. Continue using regular contraception and barrier methods like condoms for subsequent sexual activity, as emergency contraception only addresses the single incident of unprotected sex.