How Does The Morning After Pill Work? 9 Things To Know About Emergency Contraception

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For so many reasons, it's a super positive thing that emergency contraception, AKA the morning after pill, exists. But like so many aspects of reproductive healthcare, getting really thorough information on what it is, how it works, what increases or decreases effectiveness, and what might happen to your body when you take it, can be a little hard to come by. Getting all the facts about emergency contraception is super important when making informed choices about your sex life.

As for how it works when you use it after unprotected sex? According to the International Consortium for Emergency Contraception (ICEC), the primary mechanism of an emergency contraception pill is disruption of ovulation.

While other “mechanisms” of how it works have been suggested, the ICEC says, they are not well supported by data. For example, no evidence supports the theory that ECPs interfere with the implantation of a fertilized egg. And ECPs do not cause abortion of an existing pregnancy.

According to the ICEC, the two primary ECP regimens, packaged and labeled specifically for emergency contraception (EC), are: One tablet of levonorgestrel (LNG) 1.5 mg (originally presented as 2 tablets of LNG 0.75 mg each, which can safely be taken together) and one tablet of ulipristal acetate (sold as Ella in the) 30 mg or one tablet of mifepristone 10–25 mg.

There is also something called the Yuzpe combined hormonal regimen, which is using certain types of regular birth control pills as EC, guided by your healthcare provider.

As for effectiveness, the levonorgestrel regimen (like Plan B) reduces pregnancy risk by at least half, and maybe even more — possibly about 80% to 90% for one act of unprotected intercourse, according to the ICEC.

The ICEC says that ECPs are safe, and there is no situation in which the risks of using any EC regimen outweigh the benefits, despite whatever symptoms they might cause.

It’s also important to know that EC should be really accessible — and sometimes it isn’t.

“Even though Plan B and generic versions are approved for OTC status, it is still common for people to be asked for ID, or denied EC because of their age or gender, and for pharmacies to refuse to stock it on the shelf. These create barriers for people, and people should know these things should be on the shelf and are up to date on the regulatory status,” Kelly Cleland, MPA, MPH, director of the American Society for Emergency Contraception and researcher at Princeton University tells Bustle.

Take a look below to get a little more thorough on all things emergency contraception.

1. It Is Difficult To Be Certain Of Your “Non-Fertile” Days, So Timing Is Key

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While tracking your cycle is always a good thing, it is difficult, according to the ICEC, to really accurately determine whether your sexual encounter took place on a fertile or non-fertile day. Women should not decide against using an ECP because they believe the unprotected sex took place on a day they weren’t fertile, if they don’t want to get pregnant.

It's all about timing when it comes to taking EC effectively. Regardless which regimen you are using, according to the ICEC, ECPs should be taken as soon as possible after unprotected sex, and no later than five days after sexual intercourse. This is to maximize the effectiveness before ovulation happens.

"Time is the key component to the effectiveness of emergency contraception pills," Dr. Savita Ginde, physician and healthcare advocate, tells Bustle. "This family of contraception doesn’t disrupt pregnancy, therefore it must be taken prior to pregnancy occurring."

2. It Could Be Less Effective For People Above A Certain Weight

Typically, most emergency contraception is not an effective UPA for people over 175 lbs, meaning people who weigh over 175 pounds may metabolize it in a way that negatively affects its effectiveness.

The effectiveness of some pills goes down in relation to weight, Dr. Ginde says. Your healthcare provider or a pharmacist can help select the best manufacturer for you.

In a study published in Contraception, Alison Edelman, M.D. and a team at Oregon Health and Science University showed that people who had BMIs between 35.9–46.7 had about half the levels of EC in their bloodstream than people in the range below them. Other studies done at places like Columbia University and the University of Edinburgh have shown similar results. Researchers at University of Edinburgh found that the emergency contraception pill ella was more effective for women with a higher BMI than Plan B.

The very fact that it is difficult to find thorough information or definitive answers as to the effectiveness of EC is an example of medical bias against fat people. Finding a fat-positive doctor can be a place to start when getting the best medical care for your body type.

Additionally, IUDs are an example of emergency contraception that are effective regardless of weight, Dr. Ginde says. According to Planned Parenthood, you can get a copper IUD installed up to 5 days after you have unprotected sex, and it can act as EC, and then as a more long-term form of birth control.

3. There Are Common Side Effects

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If you've taken EC before, it's very likely that you might have had some side effects that came along with it.

If you've taken EC before, it's very likely that you might have had some side effects that came along with it.

"The most common side effect of EC is a temporary change in your period," Dr. Erin Burke, PhD, Modern Fertility's head of clinical research, tells Bustle. "You might have spotting, an irregular, or heavier period after taking it."

According to the ICEC, some people experience headache, tummy pain, breast tenderness, dizziness, and fatigue. The ICEC says that most people have their period within seven days of expected time, and it typically happens on average one day earlier using levonorgestrel, and two days later using UPA. A reported 24% of people said they experienced a delay of more than seven days, irregular bleeding, spotting, or nausea .

It’s also important to note that if you vomit two hours after using levonorgestrel or three hours after using UPA, you will want to take another ASAP. And additionally, since all bodies are unique, your body might react differently, so if these or any symptoms continue for more than a week, definitely see your doctor.

4. Emergency Contraception Doesn't Make You Infertile

We often hear people wondering if taking EC can make them infertile, but that's not the case, Dr. Burke says.

"Taking Plan B does not impact a woman’s fertility in the long-term, though this misconception is quite commonly held," Dr. Burke says. "We recently ran a survey that looks at information gaps in fertility, and found that 42% of women were not aware that taking hormonal birth control (for example, pill, patch, IUD) for more than five years does not reduce a woman's fertility."

5. There Are Some Drug Interactions

There are certain types of medications and supplements that do decrease the effectiveness of ECPs, according to the ICEC. Those who use hepatic CYP450 enzymes, the HIV medicines efavirenz and ritonavir, as well as certain medicines for tuberculosis and epilepsy, and herbal medicines containing St. John’s wort, might see a decrease in the effectiveness of regular doses of ECP.

“A woman using these drugs and in need of EC should be offered the Cu-IUD or, alternatively, a double dose of LNG (3 mg),” the ICEC says in a report. “In addition, the effectiveness of UPA ECPs could be reduced if progestin was taken seven days prior or is taken within five days after UPA intake. Use of UPA ECPs is not recommended in women with severe asthma treated by oral glucocorticoid or in women with severe hepatic impairment.”

If you are concerned about certain medications or supplements decreasing the effectiveness of EC, discuss proper dosage with your healthcare provider.

6. EC Does Not Protect You Against Future Cases Of Unprotected Sex

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The morning after pill is good for one use, the ICEC says, meaning that it will not protect you from pregnancy in future cases of unprotected sex. That being said, however, people should use only one ECP treatment at a time regardless of the number of prior episodes of unprotected intercourse. You don’t double dose, for example, if you had unprotected sex two times in one night, and then take an EC the next day.

Depending on the timeframe when the unprotected sex occured, talk to a healthcare provider abour which method is best for you.

“EC does stay in the body for 24 hours, so you don’t need to repeat it for 24 hours. If you need it again, even within the same cycle, that is safe,” Cleland

says. “It’s also important to not use levonorgestrel and ulipristal acetate [the different methods of EC] at the same time. You want to stick to one method, as we are not certain whether or not they counteract each other.”

7. It's Important Not To Assume EC Worked Just Because You Took It

Again, there are some factors that contribute to the effectiveness of the pill. While you don't want to get super worried after you take it, don't assume that it's automatically worked.

“It’s not foolproof. One predictor is where you are in your cycle,” Cleland says. “Timing is a really important piece. Plan B (levonorgestrel) only works until the luteinizing hormone surge has begun. Ella (UPA) works farther into your cycle, it works after the luteinizing hormone surge, but it doesn’t work if ovulation has been triggered,” Cleland says.

If you are close to ovulation, it is not going to prevent ovulation, therefore it will not prevent pregnancy.

“The reality is, most people don’t know where they are in their cycle. We always recommend, take it as soon as possible,” Cleland says. “I worry that the product labels are a little misleading, because it really depends on where you are in your cycle.”

8. Some Women Need to Be More Cautious When Taking It

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According to the ICEC, ECPs are not dangerous for any person with a uterus, and that there are no current medical restrictions.

“There are some very limited drug interactions and affect a limited population,” Cleland says.

People talk about it being a massive dose of hormones, when in reality progestin is a pretty benign hormone. And there aren’t a lot of concerns about using it in larger doses, especially because it’s not something you are doing every day. The side effects are mild and usually go away on their own.

The ICEC says that things such as “young age, [weight], personal or family history of venous thromboembolism, prior or current breast cancer, prior ectopic pregnancy, breastfeeding, migraine headaches, cardiovascular disease, liver disease, diabetes, hypertension, and prior ECP use in the same menstrual cycle,” are not reasons to avoid taking EC.

9. You Don't Want To Rely On EC As Your Go-To Birth Control Method

"Taking as often as needed is safe, however, it’s called emergency contraception for a reason," Dr. Ginde says. "You don’t want to rely on them as your go-to method of birth control."

This is in major part because of how unpleasant the side effects can be, like bleeding and nausea. They are also less effective than more consistent forms of birth control.

"If you find yourself using them more than a couple of times, talk with your provider about the many, more effective, forms of birth control that can be used on a regular basis," Dr. Ginde says.

Again, being as informed as possible when it comes to your sex life and health is both necessary and empowering. If you have any more questions about the best way to approach birth control if that needs to be a part of your plans, talk to a trusted and informed healthcare provider to begin your research.

Correction: A previous version of this article misstated how emergency contraception works. It has been updated to remove incorrect information.