Plan B One-Step is an emergency contraceptive that prevents pregnancy after unprotected sex or contraceptive failure. It is a backup method, not routine birth control. Plan B does not terminate an existing pregnancy; it prevents conception.
How Plan B Works
Plan B One-Step contains levonorgestrel, a synthetic hormone. It primarily prevents pregnancy by delaying ovulation. If no egg is released, fertilization cannot occur.
While primarily affecting ovulation, levonorgestrel can also prevent fertilization by altering sperm or egg transport. It does not prevent implantation of a fertilized egg.
The Critical Time Window
Plan B’s effectiveness depends on how quickly it is taken. It is most effective as soon as possible. While effective up to 72 hours (three days) after unprotected sex, its efficacy decreases over time.
Plan B is approximately 94-95% effective within the first 24 hours. This drops to 60-89% within the 72-hour window. Some research suggests moderate effectiveness up to 120 hours (five days).
Plan B works best before ovulation, as its main action is to delay egg release. If ovulation has already occurred, the medication may be less effective or ineffective.
Factors Influencing Effectiveness
Beyond timing, other factors affect Plan B’s effectiveness. Body weight is one consideration; research indicates reduced efficacy in individuals with higher BMI. Some studies suggest Plan B may be less effective for those over 155 pounds or with a BMI of 25-30 or higher.
Theories for reduced effectiveness include drug dilution, altered hormone metabolism, or interactions with hormones in body fat. Health organizations state Plan B still provides protection across all weight groups, though alternatives may be more suitable for some.
Certain medications can interfere with Plan B’s effectiveness. These include anti-seizure medications (carbamazepine, phenytoin, phenobarbital), antibiotics (rifampin, griseofulvin), the herbal supplement St. John’s Wort, and some HIV medications (efavirenz).
Beyond Plan B’s Window
When Plan B is not effective due to time or body weight, alternative emergency contraception methods are available. Ulipristal acetate (brand name ella) is one option. This prescription medication is effective for up to 120 hours (five days) after unprotected sex.
Ulipristal acetate delays ovulation, even if imminent. It may be more effective than levonorgestrel-based pills, especially later in the five-day window or for individuals with higher body weight.
Another highly effective emergency contraception option is the copper intrauterine device (IUD). This device can be inserted by a healthcare provider up to 120 hours (five days) after unprotected sex. The copper IUD works by creating a chemical change that prevents sperm from fertilizing an egg, making it over 99% effective.
The copper IUD offers the added benefit of providing ongoing birth control for up to 10 years once inserted. If there are concerns about potential pregnancy, consulting a healthcare provider is an important step to discuss the most appropriate emergency contraception option.