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A morning after pill failing

Illustration by Marta Pucci

Sex

Why the emergency contraception pill fails

Birth Control 101

by Sarah Toler, Science Writer for Clue Reviewed by Nicole Telfer, Science Content Producer
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Top things to know:

  • In the United States, there are two different types of emergency contraceptive pills: one is available over the counter, while the other needs a prescription

  • One-dose emergency contraception pills prevent pregnancy about 50-100% of the time 

  • Some reasons emergency contraceptive pills can fail include ovulation timing, BMI and drug interactions

Most of the time, emergency contraception works, when it’s taken within the recommended window following unprotected sex. 

There are two types of commonly used emergency contraception: emergency contraceptive pills and the copper IUD (1). In this article, we are going to focus on emergency contraceptive pills, which are also called emergency birth control or morning-after pills—we’ll use these terms interchangeably. (If you’re interested in learning more about the copper IUD, you can read about it here. Additionally, in some cases, combined oral contraceptives (“the pill”) can also be used as emergency birth control).

The effectiveness of the morning-after pill can be hard to study, because it can be difficult to know if people are ovulating, when they had sex and what day they are on in their cycle (2,3). Based on the data scientists have, one-dose emergency contraception pills are effective at preventing pregnancy about 50-100% of the time (3,4). (The range varies widely because there are many components to fertility: people don’t get pregnant every single time they have unprotected sex, and there are lots of different factors which lead to conception.) 

There are two types of pills used for emergency contraception (1), which we’ll describe in more detail, below.

Progestin emergency contraception (also known as levonorgestrel) 

Available over-the-counter

In the United States, the progestin-only emergency contraception pill can be purchased in pharmacies by anyone of any age without a prescription (1). You should take it as soon as you can after unprotected sex, but it’s most effective if taken within 72 hours (1). You can take it for up to five days after intercourse. If you take it between three and five days, the rate of effectiveness is reduced (1).

It works by delaying the release of the egg from the ovary (ovulation), preventing the sperm  from fertilizing it (5).

Antiprogestin emergency contraception pill (also known as ulipristal acetate)

Available by prescription

Antiprogestin emergency contraception pills are the most effective form of emergency contraceptive pill (1) and available only by prescription. This pill should be taken as soon as possible after unprotected sex (6), but you can take it up to five days later (1).

This emergency contraception pill changes the way progesterone works in your body (1). It works by preventing or slowing ovulation (1). When ovulation is postponed or halted, there is no egg for sperm to fertilize, and pregnancy is prevented. 

Why does emergency birth control fail?

A person who takes emergency contraception can still get pregnant. Here are the three primary reasons why this happens. 

You started ovulating before you took the pill

Emergency birth control is all about timing. It's recommended that you take the pill as soon as you can—if you wait too long, you might miss the window during which the pill can be effective.

If you take it right after sex, it can prevent you from ovulating if you haven’t started already (1). If you have sex on at the time you ovulate or after you ovulate, your emergency contraception pill won’t be effective (7). 

If you have unprotected sex again after you take the pill in the same cycle, it could also fail (7).

The morning-after pill doesn’t work as well for your body type 

Healthcare providers use the Body Mass Index (BMI) scale to group people into generic categories based on height and weight (8). 

Current studies show that emergency contraceptive pills are less effective for people who have a BMI of 30 or higher (7,9). 

People who have a BMI of 30 or higher who take emergency contraception have pregnancy rates that are 2 to 4 times higher than those with a BMI of 25 or less (7,9). But more research is needed. 

Antiprogesterone emergency contraceptive pills offer better prevention of pregnancy for those with a BMI of 30 and higher than progestin emergency contraceptive pills (7), and a copper IUD may be the most effective method of emergency birth control for those with a BMI of 30 or over (1,7). 

Though the morning-after pill working has lower efficacy rates for people with higher BMIs, it still prevents pregnancies in this group. It’s better to take it than not take it, in order to prevent pregnancy (9). 

You’re taking a medication that interacts with emergency contraception

Drugs or herbal products can both cause emergency birth control pills to be less effective (6,10). 

Take note, these drugs and herbs should not be taken with emergency contraception pills (10,6):

  • Barbiturates

  • Bosentan

  • Carbamazepine

  • Felbamate

  • Griseofulvin

  • Oxcarbazepine

  • Phenytoin

  • Rifampin

  • St. John’s wort

  • Topiramate

Be sure to double check the emergency contraceptive package information insert for more detailed information. 

Hormonal birth control can also cause interactions with the antiprogestin emergency contraceptive pill. We’ll talk more about delayed use, below.

What to do after you take emergency birth control

Progestin emergency contraception (also known as levonorgestrel) 

Available over-the-counter

After you take the progestin-only emergency contraception pill, use a barrier method like external or internal condoms or don't have sex for least seven days (1). You can continue or start taking hormonal birth control (like the pill, patch, shot, or ring), start straight away (1).

Antiprogestin emergency contraception pill (also known as ulipristal acetate)

Available by prescription

If you take the antiprogestin emergency contraceptive pill, use barrier methods or avoid having sex until your next period. If you want to start (or continue) taking hormonal birth control, wait until five days after taking the emergency contraceptive pill (1,11). (Hormonal birth control can cause the antiprogestin emergency contraceptive pill to be less effective if taken at the same time (1,11)). 

After following these precautions, you can go about living your life. You don't need any follow-up tests or procedures (1). You don't even need to tell your healthcare provider (unless you want to) (1). If your period is over a week late, take a pregnancy test to be sure you aren’t pregnant (1).

What happens if you took emergency contraception and do get pregnant? 

Emergency contraceptive pills won't harm the pregnancy or the fetus (1).

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Planning ahead

It might be a good idea to stock up on an emergency contraceptive pill in case you need one in the future. You can ask your healthcare provider for a "just-in-case" prescription or grab an over-the-counter pill from the pharmacy to keep at home.

Don't forget—the emergency contraceptive pill, like hormonal birth control, does not prevent sexually transmitted infections (STIs). If you think you might be at risk for STIs, always use a barrier method when you have sex and talk to your healthcare provider about getting an STI screening. 

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