How It Works
Plan B’s active ingredient is a synthetic hormone called levonorgestrel, a type of progestin (meaning it works like the hormone progesterone). If you take Plan B after sex, the levonorgestrel works to prevent ovulation, aka the release of the egg. Keep in mind that if you’ve already ovulated, Plan B won’t be effective — so while Plan B is recommended for use within 72 hours of having unprotected sex, you should take it "the sooner the better, because it’s going to then prevent the release of that egg," Ashton Strachan, DNP, CRNP, of the Student Health Services at University of Alabama at Birmingham, tells Refinery29. "If you wait too late, your body may release that egg before [Plan B] has the time to stop that release."
The Side Effects
Plan B’s most common side effect is that it may change your next period – your period could be up to a week early or late, heavier or lighter than usual, and you may see some spotting between taking Plan B and your next period. Other potential side effects include nausea (and occasionally vomiting), cramps, headache, dizziness, and tender breasts. These symptoms should disappear within a few days. "This is still a hormone, so any symptoms they could get when taking [hormonal birth control like] the Pill, they could potentially get taking Plan B," Strachan explains.
About The “Weight Limit”...
You may have heard about a “weight limit” for Plan B, especially if you've watched the new Hulu series Shrill. However, there’s conflicting research out there. Some studies have suggested that Plan B may be less effective for people who weigh over 165 pounds or have a BMI over 25 (keep in mind that the average American woman weighs 170.5 pounds). However, other studies have found no difference.
In a Consumer Q&A post, the FDA writes that these studies are "too conflicting and too limited to make a definitive conclusion," so "the FDA does not believe that a change in the labeling [for Plan B] is warranted at this time. The FDA continues to believe all women, regardless of how much they weigh, can use these products to prevent unintended pregnancy following unprotected sexual intercourse or contraceptive failure.”
On the other hand, Planned Parenthood resources do address weight; the page for Plan B reads, "[levonorgestrel] morning-after pills are also less effective if you have a higher BMI. If that’s the case, the copper IUD or ella are better options for you." Ella is another form of emergency contraception with a different active ingredient, ulipristal acetate, from Plan B. Some studies have suggested ella is also less effective for people with higher BMIs — though that point comes at a higher weight (196 pounds) than Plan B. Weight does not impact how well the copper IUD works.
About The “Age Limit”...
How Many Times Can You Use Plan B?
Plan B does not become less effective if you take it frequently. As Planned Parenthood puts it, people "should feel free to use the morning-after pill (also known as emergency contraception) whenever they think it’s necessary." However, taking Plan B frequently may cause your period to become irregular and unpredictable. And in the big picture, it's just not as effective as other forms of birth control, such as the pill, shot, or IUD — Plan B's website even notes that "it's not meant to be used as a regular birth control method." That's why Plan B is called "Plan B," after all — it's designed to be a back-up if your usual form of birth control falls through.