Emergency contraception contains a concentrated dose of hormones, which can affect your body in different ways.

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No one plans to take Plan B. But in those unexpected cases where you need emergency contraception—whether a condom failed, you forgot to take your birth control pills, or you simply didn't use any form of contraception—Plan B (or the generics, My Way, Take Action, and Next Choice One Dose) can provide some peace of mind.

Because it contains a highly concentrated dose of hormones to block pregnancy after sex has already occurred (unlike the birth control pill or an IUD), there are some side effects of Plan B you should be aware of before you take it. Here's the deal.

What Is Plan B and How Does It Work?

Plan B uses levonorgestrel, the same hormone found in low-dose birth control pills, explains Savita Ginde, M.D., chief healthcare officer at Stride Community Health Center in Denver, CO, and former chief medical officer of Planned Parenthood of the Rocky Mountains. "It's a type of progesterone [a sex hormone] that's been used safely in many birth control pills for a very long time," she adds.

But there's three times more levonorgestrel in Plan B compared to a regular birth control pill. This large, concentrated dose "interferes with the normal hormone patterns necessary for a pregnancy to occur, by delaying the release of an egg from the ovary, stopping fertilization, or preventing a fertilized egg from attaching to the uterus," says Dr. Ginde. (Related: What Ob-Gyns Wish Women Knew About Their Fertility)

Let's be super clear here: Plan B is not an abortion pill. "Plan B cannot prevent a pregnancy that has already happened," says Felice Gersh, M.D., an ob-gyn and founder and director of the Integrative Medical Group of Irvine, in Irvine, CA. Plan B works largely by stopping ovulation from occurring, so if it's taken right after ovulation and the potential for fertilization still exists (meaning, there's potential for that newly released egg to meet up with a sperm), Plan B could fail to prevent pregnancy. (Reminder: Sperm can chill out and wait around for an egg for about five days.)

That said, it's pretty effective if you take it within three days of having unprotected sex. Planned Parenthood says that Plan B and its generics lower your chance of getting pregnant by 75-89 percent if you take it within three days, while Dr. Gersh says, "if taken within 72 hours of the sexual encounter, Plan B is nearly 90 percent effective, and is most effective the sooner it's used."

"If you're around the time of ovulation, clearly the sooner you take the pill, the better!" she says.

The Potential Side Effects of Plan B

The side effects of Plan B are typically temporary and harmless, says Dr. Ginde—if you have any side effects at all. In one clinical trial looking at the side effects of Plan B in women:

  • 26 percent experienced menstrual changes
  • 23 percent experienced nausea
  • 18 percent experienced abdominal pain
  • 17 percent experienced fatigue
  • 17 percent experienced headaches
  • 11 percent experienced dizziness
  • 11 percent experienced breast tenderness

"These symptoms are a direct effect of the levonorgestrel, and the drug's effect on the gastrointestinal tract, the brain, and the breasts," says Dr. Gersh. "It can impact hormone receptors in various ways, resulting in these side effects."

Discussions online back this up: In a Reddit thread in the r/AskWomen subreddit, many women cited no side effects at all or, if they did have some, said they only experienced minor bleeding, cramping, nausea, or cycle irregularities. A few noted that they felt more significantly sick (ex: threw up) or had heavier or more painful periods than usual. Something important to note: If you do throw up within two hours of taking Plan B, you should talk to your healthcare professional to find out if you should repeat the dose, according to the Plan B website.

How long do Plan B Side effects last? Luckily, if you get any side effects at all, they should only last for a few days after taking it, according to the Mayo Clinic.

No matter where you are in your cycle when you take Plan B, you should still get your next period at about the normal time, says Dr. Gersh—though it could be a few days early or late. It may also be heavier or lighter than normal, and it's not abnormal to experience some spotting a few days after taking Plan B. (Related: 10 Possible Causes of Irregular Periods)

When Should You See a Doctor?

While Plan B's side effects aren't dangerous, there are a few instances where it's best to talk to your doctor to see what's up.

"If you develop bleeding for longer than a week—whether spotting or heavier—you should see a physician," says Dr. Gersh. "Severe pelvic pain also requires a visit with the doctor. If pain develops three to five weeks after taking Plan B, it may indicate a tubal pregnancy," a type of ectopic pregnancy when a fertilized egg gets stuck on its way to the uterus.

And if your period is more than two weeks late after taking Plan B, you should do a pregnancy test to determine if you might be pregnant. (Here's what you need to know about the accuracy of pregnancy tests and when to take one.)

Additional Factors to Keep In Mind

Taking Plan B is generally considered safe, even if you have a condition like polycystic ovarian syndrome (PCOS) or uterine fibroids, says Dr. Ginde.

There is some concern over the effectiveness of it in women who weigh over 175 pounds, though. "Several years ago, two studies showed that after taking Plan B, women with a BMI over 30 had half the level of Plan B in their bloodstream compared to women with normal range BMI," she explains. After the FDA reviewed the data, though, they found there was not enough evidence to force Plan B to change their safety or efficacy labeling. (Here's more info on the complicated topic of whether Plan B works for bigger-bodied people or not.)

Dr. Gersh also recommends that women with a history of migraines, depression, pulmonary embolism, prior heart attack, stroke, or uncontrolled hypertension consult their physician prior to taking it because these conditions all have the potential for hormone complications. Ideally, you'll have this conversation just in case, long before it may be needed. (Luckily, if you need to speak to a provider ASAP, telemedicine could help.)

But remember: It's called emergency contraception for a reason. Even if you don't experience any horrible side effects of Plan B, "don't rely on it as your go-to method of birth control," says Dr. Ginde. (See: How Bad Is It to Use Plan B as Birth Control?) "These pills are less effective than other forms of regular and routine birth control, and if you find yourself using them more than a couple of times, you should talk with your provider about the many (more effective) forms of birth control that can be reliably used on a regular basis."