Emergency contraception is a way to prevent pregnancy after unprotected sex or a contraceptive failure, such as a broken condom or a missed birth control pill. It works primarily by delaying or preventing ovulation, the release of an egg from the ovary. Despite the nickname “morning-after pill,” most forms can be used up to five days (120 hours) after sex, though they work better the sooner you take them.
Types of Emergency Contraception
There are three main options, and they differ in how you get them, how long you have to use them, and how well they work.
Levonorgestrel pills (Plan B One-Step and generics) are available over the counter to anyone, at any age, without a prescription. You take a single pill as soon as possible. Effectiveness is highest within the first three days, though it offers some protection through day five. Pregnancy rates increase noticeably when taken on days four and five.
Ulipristal acetate (ella) requires a prescription. It works within the same five-day window but maintains its effectiveness more consistently across all five days. It also prevents ovulation more effectively than levonorgestrel, particularly when you’re close to ovulating, which is the highest-risk time in your cycle.
IUDs (intrauterine devices) are the most effective form of emergency contraception. The copper IUD has long been used for this purpose, and a hormonal IUD releasing levonorgestrel appears to work just as well. In one study of over 600 women, the pregnancy rate with an IUD was 0.3%, compared to 1.4 to 2.6% for emergency contraceptive pills. An IUD must be placed by a healthcare provider within five days of unprotected sex, and when the timing of ovulation can be estimated, placement may be possible beyond five days as long as it occurs within five days of ovulation. The major advantage: once placed, an IUD doubles as highly effective ongoing birth control for years.
How Emergency Contraception Prevents Pregnancy
Emergency contraceptive pills work by disrupting the ovulation process. When taken before ovulation, they interfere with the hormonal signals that trigger egg release, either blocking it entirely or delaying it long enough that sperm (which survive about five days in the body) are no longer viable by the time an egg is released. This is important: emergency contraception does not end a pregnancy that has already started. The WHO states clearly that these pills do not induce an abortion.
The copper IUD works differently. Copper creates an environment in the uterus that is toxic to both sperm and eggs, preventing fertilization. The hormonal IUD also thickens cervical mucus and thins the uterine lining alongside its local hormone effects.
Effectiveness by Timing
Timing matters more for pills than for IUDs. For levonorgestrel (Plan B), effectiveness is strongest in the first 24 hours and drops gradually. A large analysis found pregnancy rates stayed low through four days but rose between days four and five. For ulipristal (ella), two studies showed consistent effectiveness across the full 120-hour window, making it the better pill option if several days have already passed.
IUDs are the clear winner regardless of timing within the five-day window. If you can get an appointment quickly enough, an IUD is the most reliable choice available.
Body Weight and Effectiveness
Levonorgestrel pills may be less effective for people who weigh more than 165 pounds or have a BMI over 25. Canada’s health regulator has gone so far as to say levonorgestrel may be ineffective for people over 176 pounds, though other experts consider this an overstatement given the limited data. Ulipristal (ella) appears to work better than levonorgestrel for people with a BMI over 30.
If your weight is a concern, the best options are ella (by prescription) or an IUD, since neither loses effectiveness at higher body weights. That said, medical organizations emphasize that people over 165 pounds should not be discouraged from using levonorgestrel if it’s the only option available. Some protection is better than none.
Side Effects and What to Expect
Emergency contraceptive pills commonly cause nausea, headache, fatigue, dizziness, breast tenderness, and stomach cramps. These side effects are generally mild and resolve within a few days. If you vomit within two hours of taking the pill, you may need to take another dose.
Your next period will likely be affected. In a study of 232 women, about 15% experienced unexpected bleeding between periods, and roughly 30% of users in broader research reported some bleeding within seven days of taking the pill. Up to 13% experienced their period arriving more than a week late. The specific changes depend on where you are in your cycle when you take it: using it early in your cycle tends to shorten that cycle and cause spotting, while use later in the cycle can delay your period.
These disruptions are temporary. In the study, the majority of changes disappeared by the following cycle. If your period is more than a week late, a pregnancy test is a reasonable next step.
How to Get It
Plan B One-Step and its generic versions sit on pharmacy shelves alongside other over-the-counter products. There are no age restrictions and no need to show ID. Two-pill generic versions of levonorgestrel are available without a prescription for people 17 and older. Ella requires a prescription, which you can get from a doctor, nurse, or family planning clinic. Some telehealth services also prescribe it. IUDs always require a provider visit for insertion.
Cost varies. Plan B generics can cost anywhere from $10 to $50 at pharmacies. Ella is typically more expensive without insurance. IUDs have higher upfront costs but are usually covered by insurance and serve as long-term contraception, which can make them more cost-effective over time.
What Emergency Contraception Does Not Do
Emergency contraception is not the same as the abortion pill. It prevents pregnancy from occurring; it does not affect an existing pregnancy. It also does not protect against sexually transmitted infections. And while it is safe to use more than once, it is not designed as a regular birth control method. Ongoing contraception is more effective and more affordable over time.