While Plan B reduces your chance of pregnancy by 75-89% if taken correctly, it does have some potential side effects, like any other form of birth control. Luckily, these side effects aren’t serious. We talked to Ashton Strachan, DNP, CRNP, of the Student Health Services at University of Alabama at Birmingham, to get the facts about Plan B side effects.
First, it’s important to understand how Plan B works. Plan B's active ingredient is levonorgestrel, which is a synthetic form of the hormone progestin. After you take Plan B, the progestin works to decrease the stimulation of two hormones: follicle stimulating hormone and luteinizing hormone. Usually, these two hormones work together to trigger the release of an egg, called ovulation. But the progestin in Plan B prevents this release in what's called negative feedback inhibition. “If it’s inhibiting those hormones, then it’s not going to stimulate that egg to release,” Strachan explains.
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All this means that Plan B works to prevent ovulation — not fertilization. So if the egg has already been released, Plan B won't be effective, and it won't end an existing pregnancy. Plan B is recommended for use within 72 hours of unprotected sex, but the sooner you take it, the better.
Now, the side effects. First, Plan B can “potentially alter when your next cycle would be,” Strachan explains. Your period may arrive up to a week earlier or later than usual, or it could not be affected at all. It also may be lighter or heavier than usual. Additionally, you may see some spotting between the time you take Plan B and your next period. Generally, your period should return to normal by your next cycle, but if you take Plan B often, your period could become irregular for a longer time. That's part of why Plan B is a type of emergency contraception — it's meant to help prevent pregnancy if your regular form of contraception falls through, not serve as your primary form of birth control.
Plan B has a few other possible side effects, none of which are serious. These include spotting, nausea (and occasionally vomiting), cramps, headache, dizziness, and tender breasts. Planned Parenthood notes that if you vomit within two hours of taking Plan B, it won’t be effective and you’ll need to take it again. Anecdotally, some people have noticed acne breakouts after taking Plan B, though it's not in the official list of side effects. Apart from potentially throwing off the timing of your next period, Plan B side effects usually last only a few days.
If this all sounds familiar, that’s because the potential side effects of taking Plan B are similar to the potential side effects of taking other forms of hormonal birth control, like the pill. Which makes sense — both contain progestin, although Plan B contains a higher dose, and the pill usually contains estrogen as well (though there are some progestin-only forms of birth control pills). ”It’s the symptoms you would potentially get if you took birth control, because this is also a form of birth control,” as Strachan puts it.
Via Plan B guidelines, if your period is more than a week late, then you should take a pregnancy test to make sure that Plan B was effective. If you experience lower abdominal pain three to five weeks after taking Plan B, seek medical attention, because that may be a sign of an ectopic pregnancy. And as always, you can ask your doctor if you have any concerns.