The most effective way to prevent pregnancy after unprotected sex is to use emergency contraception as soon as possible, ideally within the first 24 hours. You have up to five days (120 hours), but effectiveness drops with each passing day. There are four options available in the United States: three types of emergency contraceptive pills and the copper IUD.
Your Options, Ranked by Effectiveness
Not all emergency contraception works equally well. Here’s how they compare:
The copper IUD is the most effective option. When inserted by a healthcare provider within five days of unprotected sex, it prevents pregnancy more than 99% of the time. The pregnancy risk is roughly 0.1%. It also doubles as long-term birth control for up to 10 years if you choose to keep it.
A hormonal IUD is a newer but well-supported alternative. A study of over 700 participants found that the pregnancy risk with a hormonal IUD used as emergency contraception was 0.3%, statistically comparable to the copper IUD. This option is underutilized but worth asking about, especially if you’d prefer a hormonal IUD for ongoing contraception.
Ulipristal acetate (sold as ella) is a prescription-only pill effective for up to five full days after sex. The pregnancy rate in clinical studies was about 1.2%. It holds its effectiveness better across the five-day window than other pills, making it the strongest pill option if a few days have already passed.
Levonorgestrel (sold as Plan B, My Way, and other brands) is available over the counter without a prescription or age restriction. It works best within the first three days (72 hours). The pregnancy rate in studies ranged from 1.2% to 2.1%, but effectiveness drops noticeably after day three and climbs further at days four and five.
A fourth pill option, the combined estrogen-progestin regimen (called the Yuzpe method), exists but is less effective than either of the other pill types. It’s generally only used when nothing else is available.
Timing Matters More Than You Think
Emergency contraception can prevent up to 95% of pregnancies when taken within five days, but “within five days” doesn’t mean all five days are equal. For levonorgestrel pills, a large meta-analysis found that pregnancy rates stayed low through the first four days but increased substantially on days four and five. Ulipristal acetate maintained more consistent effectiveness across the full 120-hour window, which is why it’s the better pill choice if you’re past the 72-hour mark.
For either IUD type, the five-day window is firm. You’ll need an appointment with a provider who can insert it, so call as soon as you decide this is your preferred route.
How Emergency Contraception Works
Emergency contraceptive pills work primarily by delaying or stopping ovulation. If your body hasn’t released an egg yet, the pill prevents that release, and without an egg, pregnancy can’t happen. This is why timing matters so much: the pills are most effective when taken before ovulation occurs.
If ovulation has already happened, levonorgestrel pills are unlikely to help. Research consistently shows that levonorgestrel taken at or after ovulation results in conception rates similar to placebo. It does not prevent a fertilized egg from implanting, and it does not affect an existing pregnancy or fetal development.
Ulipristal acetate has a slightly broader mechanism and can delay ovulation even closer to its natural timing, which partly explains its stronger performance later in the five-day window. The copper IUD works differently from pills altogether. It creates an environment in the uterus that is toxic to sperm and eggs, which is why it remains effective regardless of where you are in your cycle.
Body Weight Affects Pill Effectiveness
This is important and not widely known. Levonorgestrel-based pills (Plan B and generics) become significantly less effective at higher body weights. Research shows that people weighing 70 kg (about 154 pounds) or more see reduced effectiveness, and at 80 kg (about 176 pounds) and above, levonorgestrel may have essentially no efficacy. The reason is pharmacological: at higher weights, the body achieves peak drug concentrations roughly 50% lower than in people under 70 kg.
People with a BMI of 30 or higher who used levonorgestrel had more than four times the risk of pregnancy compared to those with a BMI under 25. Ulipristal acetate performs better across a wider weight range, making it the preferred pill for people in higher weight categories. The copper IUD remains the most effective option regardless of weight.
If you weigh over 165 pounds and your only accessible option is an over-the-counter levonorgestrel pill, it’s still worth taking, but you should also try to get an appointment for an IUD or a prescription for ulipristal acetate as a more reliable backup.
Where to Get Emergency Contraception
Levonorgestrel pills (Plan B, My Way, and store-brand equivalents) are sold over the counter at pharmacies. No prescription, no ID, no age requirement. They typically cost between $20 and $50.
Ulipristal acetate (ella) requires a prescription. You can get one from a primary care provider, an OB-GYN, urgent care, or through telehealth services that offer same-day prescriptions with pharmacy pickup.
Either type of IUD requires a visit to a healthcare provider for insertion. Planned Parenthood clinics, OB-GYN offices, and some urgent care centers can place IUDs, sometimes on the same day. Call ahead and explain that you need emergency contraception so they can prioritize scheduling.
Side Effects and What to Expect Afterward
Emergency contraceptive pills commonly cause nausea, fatigue, headaches, breast tenderness, and abdominal cramping. These side effects are temporary, usually lasting a day or two. Some people experience light spotting after taking the pill.
The most noticeable effect is on your next period. Emergency contraception frequently shifts your cycle, making your period come earlier or later than expected. The flow may also be lighter or heavier than usual. This is normal and not a sign that something went wrong.
If your period is more than seven days late, take a pregnancy test. A missed period beyond that window is the primary sign that emergency contraception may not have worked.
Starting Regular Birth Control Afterward
If this experience has you thinking about ongoing contraception, you can start most methods right away, with one exception. After taking a levonorgestrel pill, you can begin hormonal birth control (the pill, patch, ring, implant, or injection) immediately. You don’t need to wait for your next period.
After taking ulipristal acetate, the timing is different. You need to wait six days before starting any method containing hormones. Starting a hormonal method sooner can reduce the effectiveness of both the emergency contraception and the new method. During those six days, and for the first seven days on your new hormonal method (two days if using progestin-only pills), use condoms as backup.
If you chose an IUD for emergency contraception, you already have highly effective long-term birth control in place. The copper IUD works for up to 10 years, and hormonal IUDs last three to eight years depending on the type.