Levonorgestrel tablets work by preventing ovulation, but timing and dosage matter for effectiveness

Levonorgestrel is a synthetic progestin that stops your ovaries from releasing an egg. If there is no egg, pregnancy cannot happen. The tablet works best when taken within 72 hours of unprotected intercourse—the sooner you take it, the more effective it is. Some formulations work up to 120 hours (five days), but effectiveness drops significantly after 72 hours.

Levonorgestrel does not end a pregnancy that has already started. It prevents pregnancy from beginning. If you are already pregnant when you take it, the tablet will not work and will not harm the pregnancy.

The tablet comes in different strengths: 0.75 mg (usually taken as two doses 12 hours apart, or as a single 1.5 mg dose) and 1.5 mg (taken as a single dose). Some brands are sold over the counter; others require a prescription. The single-dose version is more commonly available and easier to use correctly.

Key Takeaways

  • Levonorgestrel prevents ovulation and works best within 72 hours of unprotected intercourse, though some formulations are effective up to 120 hours.
  • The tablet is most effective when taken as soon as possible after intercourse; waiting longer reduces how well it works.
  • Your weight may affect how well the tablet works—heavier body weight can reduce effectiveness, and some doctors recommend a higher dose or a different method for people over a certain weight.
  • Levonorgestrel does not protect against sexually transmitted infections and does not replace regular contraception.
  • Certain medications, including some epilepsy drugs and herbal supplements like St. John's Wort, can interfere with how the tablet works.

How the tablet prevents pregnancy

Levonorgestrel works by thickening cervical mucus, which makes it harder for sperm to reach an egg. It also thins the uterine lining and can delay or prevent ovulation altogether. The exact mechanism depends on where you are in your menstrual cycle when you take it—it is most effective at stopping ovulation if taken before the surge of luteinizing hormone (LH) that triggers the release of an egg.

The tablet does not work if ovulation has already happened. This is why timing matters so much. If you take levonorgestrel after your egg has been released, it cannot prevent pregnancy because the mechanism that would have stopped ovulation is no longer relevant.

Timing and effectiveness rates

Within 72 hours of intercourse, levonorgestrel reduces the risk of pregnancy by 60 to 90 percent, depending on the study and the exact timing. The earlier you take it, the higher the effectiveness. A tablet taken within 12 hours is more effective than one taken at 48 hours.

After 72 hours, effectiveness drops sharply. Some formulations (particularly those containing 1.5 mg of levonorgestrel) may work up to 120 hours, but the reduction in pregnancy risk becomes much smaller. By five days after intercourse, the tablet offers minimal protection.

Effectiveness also depends on your menstrual cycle. The tablet works best in the first half of your cycle, before ovulation is likely to occur. If you are already in the second half of your cycle, ovulation may have already happened, and the tablet will not work.

Body weight and other factors that affect how well it works

Research shows that levonorgestrel is less effective in people who weigh more. Studies have found reduced effectiveness in people weighing over 165 pounds, and significantly reduced effectiveness in people weighing over 195 pounds. If you fall into these weight ranges, talk to a doctor or pharmacist about whether a higher dose or a different emergency contraception method (such as the copper IUD or ulipristal acetate tablet) might be more suitable.

Certain medications reduce how well levonorgestrel works. These include some epilepsy medications (phenytoin, phenobarbital, carbamazepine), the antibiotic rifampin, and the herbal supplement St. John's Wort. If you take any of these regularly, tell the pharmacist or doctor before taking levonorgestrel, because you may need a different dose or method.

Hormonal contraceptives you are already taking do not interfere with levonorgestrel, but they also do not add to its effectiveness. If you are on the pill, patch, or ring, levonorgestrel still works the same way.

What happens after you take the tablet

Most people experience no side effects from levonorgestrel. Some report nausea, fatigue, headache, breast tenderness, or dizziness in the hours or days after taking it. These effects are usually mild and pass within a day or two. Taking the tablet with food may reduce nausea.

Your next period may arrive a few days earlier or later than expected. This is normal. If your period is more than a week late, or if you have any signs of pregnancy (such as persistent nausea or breast tenderness beyond a few days), take a pregnancy test. A home test is reliable from the first day of a missed period.

Levonorgestrel does not cause an abortion or miscarriage if you are already pregnant. It also does not protect against sexually transmitted infections. If there is any chance you or your partner has an STI, get tested.

When to call a doctor or pharmacist

Contact a healthcare provider before taking levonorgestrel if you are already pregnant, have a history of blood clots or stroke, or take medications that interact with it (see the section on body weight and other factors above). You should also speak to someone if you have severe abdominal pain, as this could indicate an ectopic pregnancy, which levonorgestrel cannot prevent.

If you have taken levonorgestrel and your period does not arrive within three weeks, or if you have signs of pregnancy, contact your doctor. They can confirm whether you are pregnant and discuss next steps.

Levonorgestrel is a one-time tool for preventing pregnancy after unprotected intercourse. It is not a regular contraceptive and should not be used as one. If you need ongoing contraception, talk to a doctor about options that suit your health and lifestyle.

Levonorgestrel versus other emergency contraception options

Ulipristal acetate (sold as ella) is a different emergency contraceptive that works up to 120 hours after intercourse and may be more effective than levonorgestrel, especially in the later window. It requires a prescription in most places. It is also more effective in people with higher body weight.

A copper IUD (intrauterine device) inserted by a doctor within five days of unprotected intercourse is the most effective emergency contraception available—over 99 percent effective. It also provides long-term contraception if you choose to keep it in place. This requires a medical appointment and is not available everywhere when ready.

Levonorgestrel is widely available without a prescription, works quickly, and has few side effects. It is a reasonable choice if you can take it within 72 hours and do not have factors (such as high body weight or certain medications) that reduce its effectiveness.

Frequently Asked Questions

Can I take levonorgestrel if I am already on birth control?

Yes. Levonorgestrel does not interact with hormonal birth control pills, patches, rings, or shots. However, if you are on the pill and missed doses before the unprotected intercourse, your regular contraception may not be protecting you, which is why you are considering emergency contraception. After taking levonorgestrel, resume your regular birth control as scheduled.

Will levonorgestrel work if I vomit after taking it?

If you vomit within two hours of taking the tablet, contact a pharmacist or doctor. You may need to take another dose. If you vomit more than two hours after taking it, the tablet has likely been absorbed and you do not need another dose. If you have ongoing nausea or vomiting, ask about taking the tablet with food or using an anti-nausea medication first.

Does levonorgestrel protect against STIs?

No. Levonorgestrel prevents pregnancy only. It does not protect against sexually transmitted infections. If there is any chance of exposure to an STI, get tested at a clinic or with your doctor, even if you have no symptoms.

What if I take levonorgestrel and then have unprotected sex again?

Levonorgestrel protects against pregnancy from the specific act of intercourse that prompted you to take it. It does not provide ongoing contraception. If you have unprotected sex again, you are at risk of pregnancy again, and you would need to consider emergency contraception for that separate incident.

Can I buy levonorgestrel without a prescription?

In most places, levonorgestrel is sold over the counter at pharmacies without a prescription. Some formulations and brands may require a prescription depending on your location and age. Ask the pharmacist what is available where you are.