The Weight Limit On Plan B You Probably Didn't Know About

Ok, so you had unprotected sex. Luckily, there are options out there so you don't have to wake up worried by possible-pregnancy induced nightmares while you wait for your next period. According to WebMD, the Plan B pill — also known as the "morning-after pill" — is a form of emergency contraception that may help prevent pregnancy after unprotected intercourse. You can take the pill up to 72 hours after having unprotected sex and it has a 87 percent effectiveness rate. That rate is even higher if you take Plan B within 24 hours post-intercourse. 

While a lot of us are familiar with the pill's time constraints, very few are aware that Plan B also has a weight limit. According to Insider, the morning-after pill may be less effective if you weigh over 155 pounds or have a body mass index (BMI) over 25. This is surprisingly low considering Medical News Today reports the average weight of an American woman over 20 is 170 pounds.

Why is the weight limit so low?

There are theories to explain why the effectiveness of emergency contraception is reduced if you are over a certain weight. According to Endocrine News, it is believed that the pill's active ingredients can be diluted in a larger blood volume. It is also theorized that the pill's hormones could become cut off in fat cells, or the pill itself could potentially be metabolized differently in a woman who has a BMI over 25.

If you have recently had unprotected sex and are over the pill's weight limit, do not skip taking Plan B out of the assumption that it won't be effective. Staci Tanouye, M.D., a gynecologist based in Jacksonville, Florida, tells Insider to be proactive. She explains, "There are really no medical contraindications to taking emergency contraceptives, and the benefit to them is potentially higher than any potential risks — pregnancy could be a much riskier state than taking a single dose of either medication." If you have recently taken Plan B and are uncertain about its effectiveness, then consider making an appointment with your doctor.