ECP most commonly stands for emergency contraceptive pill, a medication taken after unprotected sex to prevent pregnancy. Sometimes called the “morning-after pill,” ECPs can reduce the risk of pregnancy by 72% to 90% depending on how quickly they’re taken. The term ECP can also refer to external counterpulsation, a heart therapy, though the contraceptive meaning is far more widely searched.
How Emergency Contraceptive Pills Work
ECPs prevent pregnancy primarily by delaying or blocking ovulation. The most common type contains a synthetic hormone called levonorgestrel, which blunts the hormonal surge that triggers egg release. If ovulation hasn’t happened yet, the pill essentially puts it on pause so there’s no egg available to be fertilized. It also thickens cervical mucus, making it harder for sperm to move through. Contrary to older claims, current evidence shows levonorgestrel does not significantly alter the uterine lining to prevent a fertilized egg from implanting.
A second type of ECP uses a different compound (sold under the brand name ella) that works as a selective hormone modifier. This version is effective even when taken closer to the moment of ovulation, which gives it a wider window of reliability. Both types work best when taken before ovulation occurs. Neither type will end an existing pregnancy.
Types of ECP and How They Differ
There are two main ECP options:
- Levonorgestrel pills (Plan B and generics): Available over the counter in most countries with no age restriction. Most effective within 72 hours (three days) of unprotected sex, though some benefit may extend to 120 hours. Efficacy drops from roughly 87–90% when taken within 72 hours to 72–87% in the 72-to-120-hour window.
- Ulipristal acetate (ella): Requires a prescription in many countries. Effective for up to 120 hours (five days) after unprotected sex, with less decline in effectiveness over that window. Recommended as the first-choice hormonal option due to higher overall efficacy.
Both types work significantly better the sooner you take them. Waiting even a day can meaningfully reduce their effectiveness.
What Affects Effectiveness
Timing is the single biggest factor. Beyond that, body weight plays a role that’s still being studied. Some data suggests levonorgestrel may be less effective in women who weigh more than 165 pounds or have a BMI over 25, with one meta-analysis showing obese women had a fourfold greater risk of pregnancy compared to normal-weight women using the same pill. However, the European Medicines Agency reviewed this data and concluded it was “too limited and not robust enough” to draw firm conclusions, advising women to continue using levonorgestrel regardless of weight.
For women with a BMI over 30, ulipristal acetate appears to perform better, with an unintended pregnancy rate around 2.5% compared to nearly 6% for levonorgestrel. A copper IUD inserted within five days of unprotected sex is the most effective emergency contraception option at any weight, though it requires a clinic visit.
Certain medications can also reduce ECP effectiveness. Drugs that speed up liver metabolism, including some used for epilepsy (phenytoin, carbamazepine, phenobarbital) and the herbal supplement St. John’s wort, can cause your body to break down the hormones too quickly. If you take any of these, a copper IUD may be a more reliable emergency option.
Side Effects and What to Expect
ECPs are safe for the vast majority of people, but short-term side effects are common. About 23% of women experience nausea, 18% have abdominal pain, and 17% report fatigue or headache. Vomiting occurs in roughly 6% of cases. Breast tenderness and dizziness are less frequent. These effects typically resolve within a day or two.
If you vomit within three hours of taking the pill, the CDC recommends taking another dose as soon as possible, since the medication may not have been fully absorbed. Vomiting after that three-hour window generally means the drug has already entered your system. Taking an anti-nausea medication about an hour before the ECP can help prevent vomiting, though taking it after nausea starts is unlikely to help.
Your next period may come a few days early or late. About 58% of women get their period within a few days of the expected date, and most will have it within 21 days of treatment. Some spotting in the days following the pill is normal.
Long-Term Safety
Despite persistent myths, ECPs do not cause infertility or increase cancer risk. Levonorgestrel’s safety profile has been demonstrated through decades of use and extensive study. ECPs are a single, short-lived dose of hormones, not an ongoing exposure. There are no known long-term health consequences from using emergency contraception, even if used more than once. That said, ECPs are less effective than regular contraception methods over time, so they’re not ideal as a primary strategy for preventing pregnancy.
ECP in Heart Medicine
In cardiology, ECP (or EECP, for enhanced external counterpulsation) refers to a completely different treatment. It’s a noninvasive therapy for people with chronic chest pain (angina) or heart failure who haven’t responded well to other treatments. During a session, inflatable cuffs wrapped around both legs squeeze in rhythm with the heartbeat, pushing blood back toward the heart during its resting phase. This increases blood flow to the coronary arteries without surgery. The FDA has approved EECP for both refractory angina and heart failure. It’s been shown to improve exercise tolerance, reduce chest pain symptoms, and decrease the need for nitroglycerin. Some researchers have also explored its use for conditions like erectile dysfunction and restless leg syndrome, though these remain off-label.