Emergency contraception is a backup method of preventing pregnancy after unprotected sex or contraceptive failure. It comes in two forms: pills taken by mouth and intrauterine devices (IUDs) inserted by a healthcare provider. None of these methods end an existing pregnancy. They work primarily by delaying or blocking ovulation, the release of an egg from the ovary, so that sperm present in the reproductive tract never have the chance to fertilize it.
How Emergency Contraception Pills Work
Both types of emergency contraceptive pills share the same core strategy: they interfere with ovulation. But they do it differently, and that difference matters for how well they work and how long you can wait to take them.
The most widely available option, sold as Plan B and its generics, contains levonorgestrel. It works by disrupting the hormonal signal between the brain and the ovaries that triggers egg release. If taken at least two days before that hormonal surge begins, levonorgestrel can delay or prevent ovulation entirely. The catch is that once your body has already started the surge, levonorgestrel can no longer stop the process.
The second pill option, sold as Ella, contains ulipristal acetate. It blocks progesterone receptors directly inside the ovary, shutting down the specific genes needed for the follicle to rupture and release an egg. This gives it a meaningful advantage: it can still prevent ovulation even after the hormonal surge has begun, within the first several hours of that process. That’s why Ella remains more effective later in your cycle and later in the days after sex.
Older Plan B labels once suggested the pills might also prevent a fertilized egg from implanting in the uterus. The FDA has since clarified that this claim lacked scientific evidence. At the doses used for emergency contraception, these pills work before fertilization, not after.
Types and How to Get Them
Plan B and its generic versions are available over the counter in the United States with no age restrictions, no ID requirements, and no need to ask a pharmacist. You can find them on pharmacy shelves alongside other reproductive health products. Ella requires a prescription, which you can get from a doctor, clinic, or in some cases through telehealth services.
The copper IUD is the third option. A healthcare provider inserts it into the uterus within five days of unprotected sex, and it reduces the risk of pregnancy by over 99%. It’s the most effective form of emergency contraception available, and it doubles as long-term birth control for up to 10 years afterward. The tradeoff is that it requires a clinic visit and an insertion procedure.
Effectiveness and Timing
Speed matters with emergency contraception pills. The sooner you take them, the better they work.
In clinical trials, pregnancy rates after taking levonorgestrel (Plan B) ranged from 0.6% to 3.1%. For ulipristal acetate (Ella), rates ranged from 0.9% to 2.1%. When researchers compared the two head-to-head, Ella reduced the odds of pregnancy by 65% more than Plan B when taken in the first 24 hours, and by 42% more over the first 72 hours.
Plan B is labeled for use within 72 hours (three days) of unprotected sex, though its effectiveness drops steadily over that window. Ella is effective for up to 120 hours (five days), and its performance holds up better across that full timeframe. The copper IUD also works within a five-day window and maintains its near-perfect effectiveness throughout.
Body Weight and Effectiveness
Weight can affect how well emergency contraceptive pills work, and this is worth knowing if it applies to you. Levonorgestrel may be less effective in women who weigh more than 165 pounds, and some regulators have flagged it as potentially ineffective above 176 pounds or a BMI over 25. That said, major medical organizations still recommend it as an option for women at higher weights when other methods aren’t accessible, since some protection is better than none.
Ella performs better across a wider weight range. For women with a BMI over 30, the risk of unintended pregnancy with Ella is roughly 2.5%, compared to about 6% with levonorgestrel. The copper IUD is unaffected by body weight, making it the most reliable choice regardless of size.
Side Effects and Your Next Period
Side effects from emergency contraceptive pills are typically mild and short-lived, lasting only a few days. The most common ones include nausea, headache, fatigue, dizziness, breast tenderness, and stomach cramps. Some women experience light spotting between periods or heavier bleeding during their next period.
One thing that catches people off guard: emergency contraception can delay your next period by up to a week. This doesn’t mean it failed. But if your period hasn’t arrived within three weeks of taking the pill, a pregnancy test is a reasonable next step.
Medications That Reduce Effectiveness
Both levonorgestrel and ulipristal acetate are broken down in the liver by the same enzyme pathway. Medications that speed up that pathway can lower the amount of emergency contraception circulating in your body, potentially making it less effective. These include certain seizure medications, some HIV treatments (such as efavirenz), and the herbal supplement St. John’s wort. If you take any of these regularly, the copper IUD is a more dependable emergency option since its effectiveness doesn’t depend on drug metabolism.
Choosing the Right Option
Your best choice depends on timing, access, weight, and what medications you take. If it’s been less than 24 hours and you can get to a pharmacy quickly, Plan B is a reasonable and convenient option for most women under 165 pounds. If it’s been longer, if you weigh more, or if you want the most effective pill option, Ella is the stronger choice, though you’ll need a prescription. If you can get a clinic appointment within five days, the copper IUD offers the highest protection of any emergency method and provides ongoing contraception afterward.
Emergency contraception is not designed as a routine birth control method. It’s a backup for situations like a broken condom, a missed pill, or unprotected sex. It won’t protect against sexually transmitted infections, and it won’t affect a pregnancy that has already implanted. What it does, reliably, is buy your body time by pressing pause on ovulation so that the encounter in question doesn’t result in fertilization.