Plan B (levonorgestrel) is an emergency contraceptive that has to be taken within 72 hours of unprotected sex. The Plan B medication label has no weight limit. However, several studies have shown that the pregnancy rate is higher in women with a heavier body weight. The studies showed lower effectiveness in those who weigh over 165 lbs and a lack of efficacy in those who weigh over 175 lbs, especially with a BMI (body mass index) over 30. Larger blood volume may cause medication dilution, excessive fat stores can build up the medicine inside the fat cell instead of the intended cells, or the drug may be broken down faster/slower, thus blunting the medication's effects.
Medication having a weight limit is quite common! Scientists have created most existing medicines in varying dosages and routes of administration due to the pharmacological way they work in our bodies. All medications are absorbed at different rates and utilized by other organs, creating a need for specific dosages based on height and weight. Over-the-counter medications are hard to correctly dose because they are being dispensed to many people without regulation. It is always important to check with your doctor before you take any new medications.
Sources:
Cleland, K., et al. (2014). Emergency contraception review: evidence-based recommendations for clinicians. Clinical Obstetrics and Gynecology.
Gedeon Richter, Ltd. (2017). Plan B: Highlights of prescribing information.
Government of Canada. (2014). Emergency contraceptive pills to carry warnings for reduced effectiveness in women over a certain body weight.
Jatlaoui, T.C., et al. (2016). Safety and effectiveness data for emergency contraceptive pills among women with obesity: a systematic review. Contraception.
Mapes, M. (2014). Pregnant pause: “Plan B” & overweight women
