Levonorgestrel — what it is and how it works

Ever wondered about levonorgestrel after a missed pill or unexpected sex? It’s a widely used synthetic progestin found in emergency contraception (like Plan B) and some IUDs and daily pills. It mainly works by delaying or preventing ovulation and by thickening cervical mucus so sperm can’t reach an egg.

Quick facts you need to know

Use: Levonorgestrel is used for emergency contraception and in some progestin-only birth control methods, including intrauterine devices (IUDs) and daily pills.

Timing: For emergency use, take levonorgestrel as soon as you can after unprotected sex. It’s most effective within 24 hours and still useful up to 72 hours; effectiveness drops the later you take it.

Dose: Common emergency doses are a single 1.5 mg pill or two 0.75 mg pills taken 12 hours apart, depending on the product. Follow the label or your provider’s instructions.

Availability: In many countries you can buy emergency levonorgestrel over the counter at pharmacies or online without a prescription. Rules vary, so check local regulations.

Not an abortion pill: If you’re already pregnant, levonorgestrel will not end an existing pregnancy.

Side effects are usually short and mild. You might get nausea, fatigue, headache, dizziness, breast tenderness, or irregular bleeding. Your next period could come earlier or later than usual. Serious reactions are rare—if you have severe abdominal pain, very heavy bleeding, or signs of an allergic reaction (trouble breathing, swelling), seek medical help.

Interactions: Some medicines can make levonorgestrel less effective. These include certain anti-seizure drugs, some tuberculosis meds like rifampin, and herbal products such as St. John's Wort. If you take long-term medications, ask a clinician which emergency option is best.

Long-term options: Levonorgestrel-releasing IUDs (for example, Mirena) are a different use of the same hormone. They provide effective, long-term contraception and can reduce heavy periods. If you want ongoing birth control, an IUD or daily progestin pill is a better choice than repeated emergency pills.

When to see a doctor

Talk to a healthcare provider if you use emergency levonorgestrel more than once in a cycle, if you miss your next period by more than a week, or if you have signs that worry you. Also see a clinician for ongoing contraception advice—there are safer, more reliable methods for regular use.

Bottom line: Levonorgestrel is a useful emergency option and part of several birth control choices. It’s safe for most people, but it’s not a replacement for regular contraception. If you’re unsure what fits your situation, a quick chat with a pharmacist or clinician will clear things up fast.