Levonorgestrel

Key facts

• Levonorgestrel is a progestin hormone. In a single high dose (sold as Plan B, Postinor and similar emergency contraceptives) it prevents pregnancy after unprotected sex. In smaller daily doses it is one of the hormones in many combined and progestin-only birth control pills.
• For emergency contraception, take it as soon as possible after sex, ideally within 72 hours; it works mainly by delaying or stopping ovulation, so timing matters.
• It does not work if you are already pregnant and will not end an existing pregnancy. Effectiveness also drops with body weight, with delay, and with some interacting medicines.
• Seek urgent care for severe lower abdominal pain, which can signal an ectopic pregnancy.

What Levonorgestrel treats

Levonorgestrel prevents pregnancy. As emergency contraception it is used after a condom failure, a missed regular pill, or any unprotected sex. As a component of regular contraceptive pills it prevents pregnancy with ongoing use. It does not treat or protect against sexually transmitted infections, and taken as emergency contraception it is not intended as a routine, repeated method of birth control.

How Levonorgestrel works

Levonorgestrel mimics the natural hormone progesterone. Its main effect is to stop the ovary releasing an egg or to delay that release; it also thickens cervical mucus so sperm move through it less easily. Because it works mainly before ovulation, it is far less effective once ovulation has already happened, which is why speed matters.

Before you take it

• Do not rely on it if you are already pregnant; a pregnancy test is worth doing if your next period is late or unusual.
• Tell a pharmacist or doctor if you take medicines that induce liver enzymes (some anti-epilepsy drugs, rifampicin, ritonavir) or St John's wort, since these lower blood levels of levonorgestrel and can require a different approach.
• A history of ectopic pregnancy, severe liver disease or unexplained vaginal bleeding should be discussed before use.
• It does not protect against HIV or other sexually transmitted infections.

Side effects

Common effects include nausea, tiredness, headache, breast tenderness, and a change in the timing or heaviness of your next period.

Seek urgent care for:

• Severe lower abdominal pain, which can indicate an ectopic pregnancy.
• Heavy vaginal bleeding, or no period within 3 weeks of taking it.
• Signs of pregnancy after use, since the drug can fail.

Safety essentials

• Levonorgestrel emergency contraception does not work once you are already pregnant, and its effectiveness falls the longer you wait after sex, so take it as soon as possible and never treat it as routine birth control.
• If you vomit within 2 to 3 hours of taking it, take another dose or ask a pharmacist for advice.
• Use condoms or another regular method to protect future cycles and against infection.

This page is educational and does not replace advice from a doctor or pharmacist who knows your health history.