Plan B Timing Chart: Exactly How Efficacy Drops Between Hour 1 and Hour 72

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Body mass index plays a documented role in emergency contraception pharmacokinetics. A landmark 2011 analysis published in Contraception reviewed data across multiple trials and revealed that levonorgestrel fails at significantly higher rates in individuals weighing more than 165 pounds (75 kg), or those with a BMI above 25.

For individuals weighing over 175 pounds (80 kg), levonorgestrel demonstrated pregnancy rates nearly identical to taking no emergency contraception at all. Lipophilic molecules like levonorgestrel disperse differently across larger body volumes, lowering the peak serum concentration required to suppress the pituitary gland's LH signal. While taking a double dose of levonorgestrel has been studied experimentally, major medical bodies like the American College of Obstetricians and Gynecologists (ACOG) do not officially recommend double-dosing due to a lack of conclusive safety and efficacy data. Patients above these weight thresholds require different contraceptive options.