How Plan B Works and When It Is Effective
Plan B One-Step and its generics use levonorgestrel, a synthetic progesterone, to delay or prevent ovulation. According to the FDA label and the manufacturer Teva Pharmaceuticals, the pill is designed to work primarily before an egg is released. It is most effective when taken as soon as possible after unprotected sex, with labeled efficacy up to 72 hours, though some studies show reduced but still meaningful protection up to 120 hours.
Real-world effectiveness depends heavily on timing and individual biology. In a 2023 Cochrane review of emergency contraception, levonorgestrel pills reduced the risk of pregnancy after unprotected sex, but the exact percentage varied by study design and population. The review noted that no method is 100% effective, and Plan B does not terminate an existing pregnancy.
Factors That Reduce Plan B Effectiveness
Body Weight and BMI
Multiple studies and product labels indicate that levonorgestrel emergency contraception may be less effective for individuals with a body mass index of 25 or higher, and some data suggest reduced efficacy at weights above 165 pounds. In 2023, FDA labeling and product inserts continue to list weight-related limitations, and some health agencies recommend the copper IUD or ulipristal acetate as alternatives for higher-weight patients.
Medications and Health Conditions
Certain drugs, including some anti-seizure medications and the herbal supplement St. John's Wort, can lower levonorgestrel levels by speeding liver metabolism. Gastrointestinal vomiting within two hours of taking the pill may also reduce absorption, requiring a repeat dose. Patients should check with a pharmacist or prescriber about drug interactions before relying on Plan B.
Comparing Plan B to Other Emergency Contraception Options
Plan B vs. Ella and the Copper IUD
Ulipristal acetate, sold under the brand name Ella, is a prescription pill that maintains effectiveness for up to 120 hours and may work better than levonorgestrel for individuals with higher body weight. The copper IUD, inserted by a clinician within five days of unprotected sex, is the most effective form of emergency contraception and can also provide ongoing birth control for up to 10 years, according to the American College of Obstetricians and Gynecologists.
Ongoing Access and Cost
In the United States, Plan B is available without age restriction behind the pharmacy counter, and generic versions are often covered by insurance or sold for under $50 at major retailers. The SEC filings of retail and pharmacy companies show consistent availability, though supply chain disruptions can occasionally affect local stock. For long-term pregnancy prevention, health providers recommend discussing regular contraceptive methods, such as IUDs, implants, or daily pills, rather than relying on emergency options.