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How well does emergency contraception work?

Understanding EC effectiveness can get a little complicated, but if you're up to the challenge, here are the details.

by Kelly Cleland, MPA, MPH.

Emergency contraception (EC) is a great way to bounce back after a “whoops” moment and generally back up your birth control. A growing number of people have used EC, but there are still some misconceptions out there. One area of confusion has to do with effectiveness. Effectiveness for EC isn’t measured the same way as for other methods—and not all kinds of EC are equally effective. Let’s get into the nitty gritty about how well EC works—including the non-hormonal IUD and EC pills.

Non-hormonal IUDs for EC

The non-hormonal copper IUD is, by far, the most effective EC. A review of over 40 studies showed that a non-hormonal IUD inserted within 5 days of sex is 99.99% effective at preventing pregnancy. (Some studies show it works well any time after unprotected sex so long as a pregnancy test is negative.) If you get the non-hormonal IUD for EC and decide to keep using it, you’re covered for birth control for as long as you like, up to 12 years. If you need EC and you’re interested in an IUD, you can’t beat this option for effectiveness.

Levonorgestrel vs. ulipristal acetate pills

EC pills work by delaying ovulation. Levonorgestrel EC pills are available in the aisles of drug stores and can be ordered online. You may know them as Plan B One-Step, My Way, Next Choice, or After Pill. Levonorgestrel EC pills are fairly effective, but ella works even better. Ella contains a drug called ulipristal acetate (UPA), which is better than levonorgestrel at delaying ovulation.

Here's why: A spike of luteinizing hormone (LH) is what triggers ovulation. Levonorgestrel and UPA EC pills both work by slowing down LH production. UPA can delay ovulation even after the LH levels have started to rise. Levonorgestrel, on the other hand, doesn’t work once LH levels start rising. All of this is why it’s a good idea to take EC pills as soon as possible. The sooner you take it, the better the chance you can stop ovulation.

There’s some evidence that levonorgestrel EC pills might not work as well for people who weigh 165 pounds or more, and might be totally ineffective for those who weigh 176 pounds or more. When you consider that the average U.S. woman weighs 166 pounds, that may seem like cause for concern. But other studies haven’t found the same relationship between weight and levonorgestrel EC effectiveness. Right now, researchers and health care providers don’t all agree about how to interpret these studies, so if you weigh over 165 pounds and the IUD or ella aren’t an option for you, don’t write off levonorgestrel EC.

The bottom line

If you’ve had unprotected sex and need EC now, here are the steps you can take.

Step 1. If you’re open to using an IUD, go for the non-hormonal IUD. Call your health care provider or clinic and tell them that you need an appointment to get an IUD for emergency contraception. Hopefully they’ll find a way to fit you in ASAP. If they can’t, look for another clinic that can.

Step 2. The next best option is UPA EC pills. You have a couple of options for getting ella:

  • Call your health care provider or a family planning clinic and ask if you can get a prescription ASAP. Ask if they can fill your prescription. If not, ask what local pharmacies stock ella and call ahead to make sure it’s in stock.

  • Use Kwik Med to get a prescription over the phone and have the pills express mailed. Paying for the shipping can make this a more expensive option. You can lower the cost by asking Kwik Med to send your prescription to a local pharmacy that has ella in stock.

Step 3. If all of that sounds like it’s just not going to happen, go to Target, CVS, or wherever medicines are sold, and get a levonorgestrel EC pill. By the way, levonorgestrel EC pills are available without prescription for people of any age—including males. It should be right on the shelf—look in the family planning section next to the condoms. If you don’t find it on the shelf or have problems buying it, here is a handy guide to changing this by educating your pharmacist!

Why figuring out effectiveness for EC pills is messy

Figuring out how effective a method of birth control is is like a school math problem. You take a group of women who are using a method of birth control and follow them for a year. You track how many women got pregnant during that year. Then you essentially add up all women who didn’t get pregnant and divide by all the women in the study to calculate the birth control effectiveness.

So why can’t we do this kind of straightforward math for EC pills? The main reason is that EC pills are good only for one act of sex. Here’s why that messes with the math:

We know from studies that this sometimes happens:

  • You have a whoops moment, you take EC, then you have another whoops moment within a few days, and you don’t take EC again. When that happens, it’s hard to say whether any pregnancies are the result of whoops moment one or two.
  • It’s also pretty hard to know whether a woman would have gotten pregnant from a particular act of sex…did she avoid pregnancy because she took EC, or because she wouldn’t have gotten pregnant anyway? So researchers have to make a best guess. They use other information about the chances of pregnancy to estimate how many women they would expect to get pregnant, and then compare that to how many actually got pregnant.

Although researchers are using the best available information, it’s not as precise as they might like.


Kelly Cleland is a researcher at Princeton University, where her work focuses on emergency contraception and medical abortion. Kelly is also the Executive Director of the American Society for Emergency Contraception (ASEC), which promotes access to and information about EC in the United States. Before coming to Princeton, Kelly worked at the University of California, San Francisco and Planned Parenthood of New York City and got her MPA and MPH at Columbia University. Before all the serious stuff, Kelly spent her early 20s teaching English in the Czech Republic and Vietnam and wandering around the world. Nowadays, Kelly and her family are passionate rescuers of retired racing Greyhounds, so their house is often full of tall, skinny dogs who like to nap.

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