Every year, over 1.5 million people in the UK use levonorgestrel as emergency contraception. It’s one of the most common ways to prevent pregnancy after unprotected sex or contraceptive failure. But despite how widely it’s used, many still don’t understand how it actually works-or what to expect when they take it.

What Is Levonorgestrel?

Levonorgestrel is a synthetic hormone that mimics progesterone, a natural hormone in your body that regulates your menstrual cycle. It’s the active ingredient in emergency contraceptive pills like Plan B One-Step, NorLevo, and My Way. Unlike the abortion pill (mifepristone), levonorgestrel doesn’t end an existing pregnancy. It works to prevent one from starting.

The pill contains 1.5 milligrams of levonorgestrel, taken as a single dose. Some older versions required two doses 12 hours apart, but those are no longer recommended. The single-dose version is just as effective and easier to use.

How Does It Work?

Levonorgestrel doesn’t cause a miscarriage. If you’re already pregnant, it won’t affect the embryo. Instead, it delays or stops ovulation-the release of an egg from your ovary. If no egg is released, sperm can’t fertilize anything.

If ovulation has already happened, levonorgestrel may also thicken cervical mucus, making it harder for sperm to reach the egg. In rare cases, it might change the lining of the uterus so a fertilized egg can’t implant. But this last mechanism is not the main way it works, and research shows it’s unlikely to be effective if ovulation has already occurred.

Timing matters. The sooner you take it, the better. It’s most effective within 24 hours of unprotected sex. After that, its effectiveness drops steadily. By 72 hours, it’s only about 50% effective. Even though it’s approved for use up to 120 hours (five days) after sex, waiting that long isn’t reliable.

How Effective Is It?

Studies show that levonorgestrel reduces the risk of pregnancy by 85% to 95% if taken within 24 hours. That means out of 100 women who have unprotected sex and take the pill within a day, only 1 to 2 will get pregnant.

But effectiveness drops with time. If taken between 48 and 72 hours after sex, the chance of pregnancy rises to about 4%. After 72 hours, it’s no longer recommended as the best option-ulipristal acetate (ella) or an IUD are more effective.

It’s also less effective in people with higher body weight. Research from the UK’s Faculty of Sexual and Reproductive Healthcare shows that for people with a BMI over 26, levonorgestrel’s effectiveness decreases significantly. For those with a BMI over 30, it may not work at all. In those cases, an IUD is the preferred emergency option.

Side Effects and What to Expect

Most people don’t have serious side effects. The most common ones are mild and short-lived:

• Nausea (about 20% of users)
• Headache
• Tiredness
• Dizziness
• Breast tenderness
• Irregular bleeding or spotting

Nausea is the most talked-about side effect, but vomiting is rare-happening in fewer than 5% of cases. If you vomit within two hours of taking the pill, you should contact a healthcare provider. You might need to take another dose.

Your period might come earlier or later than expected. It’s normal for it to be up to a week late. If your period is more than a week late, take a pregnancy test. There’s a small chance the pill didn’t work.

Who Can Take It?

Almost anyone can take levonorgestrel. There are no age restrictions in the UK. You don’t need a prescription-it’s available over the counter at pharmacies, and some sexual health clinics give it out for free.

It’s safe for people with a history of blood clots, heart disease, or migraines with aura. Unlike combined hormonal contraceptives, levonorgestrel doesn’t contain estrogen, so it’s safer for people who can’t take estrogen-based pills.

It’s also safe to use while breastfeeding. The amount that passes into breast milk is tiny and won’t harm your baby. You might want to wait a few hours after taking the pill before feeding, but it’s not required.

What It Doesn’t Do

Levonorgestrel won’t protect you from sexually transmitted infections. If you had unprotected sex, get tested for chlamydia, gonorrhea, HIV, and other STIs. Many clinics offer free testing.

It’s not a regular birth control method. Taking it more than once in a cycle won’t increase protection-it can disrupt your cycle and make you more likely to get pregnant later.

And it won’t work if you’re already pregnant. If you took it after missing a birth control pill and later found out you were pregnant, don’t panic. Levonorgestrel doesn’t cause birth defects. The pregnancy should continue normally.

Alternatives to Levonorgestrel

There are two other emergency contraception options:

• Ulipristal acetate (ella): A prescription-only pill that works up to 120 hours after sex. It’s more effective than levonorgestrel, especially for people with higher body weight. But it’s harder to get quickly.
• Copper IUD: The most effective form of emergency contraception. It’s over 99% effective if inserted within five days. It also works as long-term birth control for up to 10 years. If you’re considering ongoing contraception, this is the best option.

Many people don’t know about the IUD as emergency contraception. If you’re eligible and want to avoid future unplanned pregnancies, talk to a clinician about getting one inserted at the same time you get the morning-after pill.

Where to Get It

In the UK, you can get levonorgestrel for free from:

• Sexual health clinics
• GP surgeries
• NHS walk-in centres

Or you can buy it over the counter at any pharmacy. Prices vary, but it usually costs between £10 and £20. Some pharmacies offer it for free if you’re under 25 or on certain benefits.

Pharmacists are trained to answer questions. You don’t need to feel embarrassed. They’ve seen it all. If you’re unsure whether it’s right for you, ask them.

When to Follow Up

After taking levonorgestrel, keep track of your cycle. If your period is more than a week late, take a pregnancy test. If it’s negative and you still haven’t gotten your period, see a doctor.

If you had unprotected sex and didn’t use emergency contraception, consider getting an STI test. Many infections don’t show symptoms right away.

Think about your long-term contraception. Emergency pills don’t protect you going forward. Talk to your GP or a sexual health clinic about options that fit your lifestyle-whether it’s the pill, IUD, implant, or condoms.

Myths vs Facts

• Myth: You need to take it within 12 hours to work. Fact: It works up to 72 hours, but sooner is better.
• Myth: It causes abortion. Fact: It prevents pregnancy before it starts. It doesn’t affect an existing pregnancy.
• Myth: It’s dangerous if you take it more than once. Fact: It’s safe to use more than once, but it’s not meant to be your regular birth control.
• Myth: Only young people use it. Fact: People of all ages use emergency contraception-including those in their 40s and 50s.

Levonorgestrel is a tool-not a failure. If you needed it, that doesn’t mean you did anything wrong. It means you took control of your body when something unexpected happened.