How Long After Unprotected Sex Does Plan B Work
Plan B One-Step and its generics are approved for use up to 72 hours after unprotected intercourse, but efficacy declines significantly after the first 24 hours. The drug is a single 1.5 mg dose of levonorgestrel that delays or prevents ovulation. According to the manufacturer's labeling and the FDA, the pill is most effective when taken as soon as possible. Studies cited by the manufacturer show a pregnancy rate of about 0.6% to 1.0% when taken within 24 hours, rising to roughly 2.5% to 3% by 72 hours. The product is available without a prescription at pharmacies, including major chains like CVS and Walgreens, and can also be purchased online through licensed pharmacies. For current pricing and availability, check the official Plan B website at PlanBOneStep.com.
The FDA label explicitly states that Plan B is not intended as a routine form of contraception and is less effective in individuals with a body mass index (BMI) of 30 or higher. The European Medicines Agency and some clinical guidelines note that the drug may not reliably prevent pregnancy in people weighing over 165 pounds, and its effectiveness drops further for those over 175 pounds. This weight-related efficacy gap means Plan B may not work at all for some individuals within the labeled 72-hour window. The FDA label for Plan B One-Step is available at FDA.gov.
When Plan B Stops Working: The 72-Hour Limit and Beyond
After 72 hours, Plan B is no longer considered effective, and the FDA does not approve its use beyond that window. The drug works primarily by delaying ovulation, so it cannot prevent pregnancy if the egg has already been released and fertilized. Emergency contraception guidelines from the American College of Obstetricians and Gynecologists confirm that ulipristal acetate (ella) remains effective up to 120 hours, but Plan B does not. If more than 72 hours have passed, a copper intrauterine device (IUD) is the most effective emergency contraception and can be inserted up to 5 days after unprotected sex. The copper IUD is over 99% effective as emergency contraception, according to ACOG clinical guidance.
Plan B does not terminate an existing pregnancy and will not work if implantation has already occurred. The drug is not the same as the abortion pill, which uses mifepristone and misoprostol. Some people mistakenly believe that taking more than one dose increases effectiveness, but the FDA-approved regimen is a single 1.5 mg dose. Doubling the dose does not extend the window of effectiveness and can increase side effects such as nausea, fatigue, and headache. The World Health Organization and the FDA both state that a second dose is not recommended for Plan B One-Step.
What to Use Instead When Plan B Is No Longer an Option
If more than 72 hours have passed, the primary alternatives are ella (ulipristal acetate) and the copper IUD. Ella is a prescription-only pill effective up to 120 hours and works by blocking progesterone. It remains effective regardless of weight, unlike levonorgestrel-based emergency contraception. Ella requires a prescription, which can be obtained through telehealth services such as Nurx or SimpleHealth, or in person at a clinic. The copper IUD, sold under the brand name Paragard, is the most effective form of emergency contraception and can be inserted by a healthcare provider up to 5 days after unprotected sex. It also provides ongoing reversible contraception for up to 10 years