What Is the Morning After Pill and How Does It Work?

The morning-after pill is a hormone-based emergency contraceptive taken after unprotected sex to prevent pregnancy, primarily by delaying or blocking ovulation so that sperm and egg never meet. It is not an abortion pill and does not end an existing pregnancy. Two main types are widely available: one containing levonorgestrel (sold under brand names like Plan B One-Step) and one containing ulipristal acetate (sold as ella). How well either works depends heavily on when you take it relative to your cycle and how soon after sex you get to it, which is why the details matter more than most people realize.

How the Morning-After Pill Prevents Pregnancy

The core mechanism is straightforward: the pill delivers a burst of synthetic hormone that interferes with ovulation. If your body hasn’t released an egg yet, the drug can delay or suppress that release long enough for any sperm already present to die off, since sperm survive roughly five days in the reproductive tract. No egg, no fertilization, no pregnancy.

Levonorgestrel, the active ingredient in Plan B and its generics, is a synthetic progestin. Research confirms it works by inhibiting ovulation to prevent fertilization.¹ However, its ability to do this depends on timing within your cycle. If the hormonal surge that triggers ovulation (the LH surge) has already begun, levonorgestrel becomes much less effective. One systematic review found that when levonorgestrel was given during or after the LH surge, pregnancy rates climbed to levels similar to unprotected cycles with no treatment at all.²

Ulipristal acetate (ella) works differently. It’s a selective progesterone receptor modulator, meaning it blocks the action of progesterone at key receptors. This gives it a wider window of effectiveness: studies show it can still inhibit ovulation even when administered shortly before the LH peak, a point at which levonorgestrel has largely lost its punch.³ That said, neither drug works once the LH peak has already occurred. Pooled data from three randomized trials found that none of the emergency contraception treatments tested were effective when given on the day of the LH peak itself.⁴

Does It Affect Implantation or Sperm?

A common concern is whether the morning-after pill could prevent a fertilized egg from implanting in the uterus, which would make it function after conception rather than before. The scientific evidence on this has shifted considerably over the years. One review of levonorgestrel’s mechanism estimated that its ovulatory inhibition potential could prevent fewer than 15 percent of potential conceptions, which led the authors to question whether a purely pre-fertilization mechanism fully explains the drug’s observed effectiveness.⁵ However, other reviews looking at levonorgestrel’s effect on implantation and fertility have concluded that the drug works by inhibiting ovulation to prevent fertilization, without strong evidence of a post-fertilization effect in practice.¹

What about effects on sperm? Lab studies have shown that levonorgestrel can slow sperm movement and slightly reduce their ability to bind to an egg, but only at concentrations far higher than what the body actually reaches after taking the pill. At the doses used in real life, the contribution of these sperm effects to emergency contraception is unlikely to be significant.⁶ So the practical story remains: the morning-after pill’s main job is stopping ovulation, not attacking sperm or blocking implantation.

The Timing Window and How Effectiveness Drops

You’ve probably heard “the sooner, the better,” and the data back that up clearly. For levonorgestrel, one large study found a pregnancy rate of about 0.8% when taken within 72 hours of unprotected sex, with an effectiveness rate between 87% and 90%. When taken between 72 and 120 hours (three to five days), the pregnancy rate rose to 1.8% and effectiveness dropped to somewhere between 72% and 87%.⁷ The pill still works during that later window, but less reliably.

Ulipristal acetate has regulatory approval for use up to 120 hours (five days) after unprotected sex and maintains higher effectiveness later in that window compared to levonorgestrel. If you’re closer to the five-day mark, ella is generally the better pharmacological option. But neither pill is a sure thing at any point: effectiveness is a probability, not a guarantee, and it depends partly on where you are in your menstrual cycle when you take it.

Body Weight and Effectiveness

This is an area where the evidence is genuinely messy, and it matters because it affects real decisions. Research has consistently found a link between higher body weight and reduced effectiveness of levonorgestrel emergency contraception. One analysis found that the estimated pregnancy rate jumped from about 1.4% among women weighing 65 to 75 kg to roughly 6% or higher for those above 80 kg.⁸ Statistical modeling in that study identified a steep increase in pregnancy risk starting around 70 to 75 kg.

A pooled analysis of four WHO studies painted a somewhat more nuanced picture. The overall pregnancy rate was low at 1.2%, and even among women over 80 kg, it was just 0.7%. However, for women with a BMI over 30, efficacy did decrease significantly. The catch: much of that finding was driven by pregnancies at a single study site, and when that site was excluded, the obesity effect disappeared. But the remaining studies also included too few women with obesity to confidently rule out a substantial drop in effectiveness.⁹

What does this mean practically? If you weigh more than about 75 kg and need emergency contraception, it’s worth discussing ulipristal acetate or a copper IUD with a pharmacist or clinician rather than defaulting to levonorgestrel. Ulipristal acetate appears to hold its effectiveness somewhat better across higher body weights, though the data are not airtight for any pill-based option in this group.

The Yuzpe Method and Why It Still Exists

Before dedicated emergency contraceptive pills were marketed, the standard approach was the Yuzpe regimen: taking a specific dose of ordinary combined oral contraceptive pills (containing both estrogen and progestin) in two doses 12 hours apart, with the first dose within 72 hours of unprotected sex. Estimates put this method’s effectiveness at reducing pregnancy risk by about 75%, with a confidence interval stretching from roughly 66% to 82%.¹⁰

That’s substantially less effective than levonorgestrel or ulipristal acetate, and it tends to cause more nausea and vomiting because of the estrogen component. So why mention it? Because in some settings, dedicated emergency contraception pills are unavailable or unaffordable, and regular birth control pills are already in someone’s medicine cabinet. It’s a backup option, not a first choice. Research into its mechanism reached similar conclusions to studies on levonorgestrel: ovulation disruption appears to be the primary way it works, with implantation interference unlikely to be the main mechanism.¹¹

Side Effects and What to Expect Afterward

The most common side effects of levonorgestrel emergency contraception are nausea, headache, fatigue, and changes to your next period. A study of 232 participants found that about 15% experienced intermenstrual bleeding (spotting between periods) and that menstrual cycle length, period duration, and the appearance of menstrual flow all shifted compared to baseline. The timing of these changes depended on where in the cycle the pill was taken. Reassuringly, the majority of these disruptions disappeared by the following cycle.¹²

So if your period comes early, late, heavier, or lighter than usual in the month you take the pill, that’s expected. If your period is more than a week late, though, a pregnancy test is a good idea, because no emergency contraceptive method is 100% effective.

Ulipristal acetate has a similar side-effect profile. Neither drug has been associated with long-term health risks. A systematic review looking specifically at the safety of repeated use of emergency contraceptive pills, even within the same menstrual cycle, found limited but reassuring evidence: the data overall did not suggest safety concerns with repeated use.¹³ That doesn’t mean using it as your routine method is a great plan, since it’s less effective than regular contraception and more expensive per use, but the fear that repeated use is dangerous is not well-supported.

Drug Interactions That Can Undermine the Pill

One risk that rarely comes up in pharmacy-counter conversations is drug interactions. Both levonorgestrel and ulipristal acetate are broken down in the liver by an enzyme called CYP3A4. Medications that rev up this enzyme, known as CYP3A4 inducers, can cause your body to clear the emergency contraceptive pill too quickly, reducing blood levels and potentially undermining its effectiveness.

A pharmacological review found that five known CYP3A4 inducers confirmed a risk of underexposure to levonorgestrel when taken at the same time. Ulipristal acetate was even more vulnerable: about 86% of its breakdown in the body is handled by CYP3A4, making it a highly sensitive substrate. Moderate to severe interactions were predicted with 17 different CYP3A4-inducing drugs.¹⁴

Common CYP3A4 inducers include certain anti-seizure medications, rifampicin (used for tuberculosis), and the herbal supplement St. John’s wort. If you take any of these regularly, a pill-based emergency contraceptive may not give you full protection, and a copper IUD is a more reliable alternative.

Ectopic Pregnancy Concerns

Some case reports have raised the question of whether levonorgestrel emergency contraception increases the risk of ectopic pregnancy, where a fertilized egg implants outside the uterus. One paper discussed how high levonorgestrel concentrations could theoretically decrease ciliary activity and tube motility, but noted that further studies would be needed to establish an actual risk.¹⁵

A systematic review addressed this more directly and concluded that the rate of ectopic pregnancy when emergency contraceptive pills fail does not exceed the rate in the general population. Since the pills reduce the overall chance of pregnancy, their use should actually reduce the absolute chance that any given act of intercourse results in an ectopic pregnancy.¹⁶

A large multicenter case-control study added some texture to this. It found that compared to non-users, women using levonorgestrel emergency contraception had a reduced risk of both ectopic and intrauterine pregnancy overall. But among women whose emergency contraception failed and who became pregnant anyway, the adjusted odds of that pregnancy being ectopic were more than five times higher than in non-users.¹⁷ The key distinction: the pill doesn’t cause ectopic pregnancies, but when it fails, the pregnancies that do occur have a disproportionate chance of being ectopic. This means that if you take emergency contraception and still miss a period, it’s important to get checked out promptly rather than assuming everything is fine.

The Copper IUD as Emergency Contraception

Pills aren’t the only option. A copper intrauterine device inserted within five days of unprotected sex is the single most effective form of emergency contraception available. In a study comparing copper IUDs to levonorgestrel IUDs for emergency use, none of the 318 participants who received a copper IUD became pregnant, representing 100% prevention of expected pregnancies. Levonorgestrel IUDs, by comparison, prevented between 93% and 96% of expected pregnancies.¹⁸

The copper IUD has several advantages beyond raw effectiveness. It’s not affected by body weight. It doesn’t rely on ovulation timing. And once inserted, it doubles as long-term contraception for up to ten years. The downsides are that it requires a clinic visit and insertion by a trained provider, which is harder to arrange on short notice than walking into a pharmacy. It can also cause heavier or more painful periods in the months after insertion, though this tends to improve over time.

Buying the Morning-After Pill and Access Barriers

In the United States, levonorgestrel-based emergency contraception has been available without a prescription since 2013, when the FDA removed age restrictions on over-the-counter sales.¹⁹ In theory, anyone can buy it off the shelf at a pharmacy regardless of age or sex. In practice, barriers persist. That same study found that while a majority of pharmacies stocked emergency contraception, disparities in access for adolescents continued even after the regulatory changes meant to fix them.

Ulipristal acetate (ella) still requires a prescription in the U.S., which adds a time barrier since the drug works best when taken promptly. In many European countries, both types have been available from pharmacies without a prescription for longer, and research from France, Norway, Sweden, and Portugal found that pharmacy access facilitated the rapid use that makes emergency contraception most effective.²⁰

Cost is another real hurdle. Levonorgestrel generics typically run between $10 and $50 at U.S. pharmacies, with brand-name Plan B often at the upper end. Ella tends to be more expensive, and the added cost of a telehealth or clinic visit for the prescription pushes the price higher. For people with limited incomes, these costs can be enough to delay or prevent use entirely. A qualitative study of low-income women found that unaffordable copays, clinic visit requirements, and restrictions on purchasing contraceptives were significant deterrents to consistent use.²¹

Can Men Buy It?

Yes. There is no legal restriction on who can purchase levonorgestrel emergency contraception over the counter in the U.S. But awareness of this is low, and the experience of actually doing it can be complicated. A survey in Providence, Rhode Island found that men were less likely than women to know about emergency contraception in the first place. Many men expressed willingness to buy it: about 56% felt they should offer to purchase it if needed, and roughly 68% believed their purchase would help prevent an unplanned pregnancy. Around three-quarters of the women surveyed agreed that men should always have access to buy it.²²

The bigger problem turns out to be misinformation at the pharmacy counter itself. A study in which male and female callers phoned pharmacies to ask about purchasing Plan B One-Step found that pharmacy employees provided incorrect information about age restrictions 51% of the time. While only seven employees specifically told the male caller he couldn’t buy the medication, the male caller was far more likely to be transferred to another person when requesting it, suggesting that staff were less comfortable handling the request from a man.²³

Does Wider Access Reduce Unintended Pregnancy Rates?

You might expect that making emergency contraception easier to get would lower unintended pregnancy rates at the population level. The evidence on this is surprisingly underwhelming. A systematic review of studies comparing advance provision of emergency contraceptive pills (giving women a supply to keep at home) to standard provision (getting them at a pharmacy after the fact) found no significant reduction in unintended pregnancies over 12 months. Patterns of regular contraceptive use, pregnancy rates, and sexually transmitted infection rates did not differ between groups in most studies.²⁴

Similarly, an observational study looking at what happened when the U.S. shifted levonorgestrel from prescription-only to over-the-counter status found that the change did not measurably impact the unintended pregnancy rate in the population studied.²⁵ This doesn’t mean easier access is pointless. It means emergency contraception is a backup method, not a population-level solution to unintended pregnancy. Access matters enormously for the individual woman who needs it on a specific night, even if it doesn’t move the needle on aggregate statistics. The professional consensus reflected in pharmacology and pediatric literature has consistently supported removing barriers to access based on safety and efficacy data, independent of the population-level pregnancy rate question.²⁶