Plan B is not abortion. Plan B (levonorgestrel) is an emergency contraceptive that prevents pregnancy from happening in the first place, primarily by delaying or blocking ovulation. Abortion terminates an existing pregnancy. These are fundamentally different biological processes, and the two are classified differently by the FDA, medical organizations, and federal law.
That said, this question comes up constantly, and the confusion isn’t random. It stems from outdated product labels, political debate, and genuine disagreements about when pregnancy begins. Here’s what the science actually shows.
How Plan B Works
Plan B contains a single high dose of levonorgestrel, a synthetic form of progesterone. Its primary mechanism is straightforward: it delays or prevents ovulation. If your body hasn’t released an egg yet, sperm have nothing to fertilize, and pregnancy can’t occur. The FDA reviewed the evidence and concluded that “Plan B One-Step prevents pregnancy by acting on ovulation, which occurs well before implantation.” The agency also stated plainly that “evidence does not support that the drug affects implantation or maintenance of a pregnancy after implantation, therefore it does not terminate a pregnancy.”
This is a key point. For years, Plan B’s own label suggested it might prevent a fertilized egg from implanting in the uterus. That language was removed after the FDA determined it wasn’t supported by the best available science. The updated consumer information leaflet no longer references effects on fertilization or implantation.
If you’ve already ovulated and an egg has been fertilized, Plan B will not disrupt that process. It won’t end an existing pregnancy, and it won’t harm a developing embryo. This is what separates it from abortion medication.
Plan B vs. the Abortion Pill
The abortion pill (mifepristone, often combined with misoprostol) works in an entirely different way. Mifepristone blocks progesterone, a hormone the body needs to maintain a pregnancy. Without progesterone, the uterine lining breaks down and the pregnancy ends. Misoprostol then causes the uterus to contract and expel its contents. This process terminates a pregnancy that has already been established.
Plan B does none of this. It acts before pregnancy begins, not after. A useful way to think about it: Plan B is closer to a regular birth control pill taken in a higher dose at a specific time, while the abortion pill is a completely different class of medication designed to end a pregnancy that’s already underway.
Why the Confusion Exists
The debate centers on the definition of when pregnancy starts. The American College of Obstetricians and Gynecologists and most of mainstream medicine define pregnancy as beginning at implantation, when a fertilized egg attaches to the uterine wall. This typically happens six to twelve days after fertilization. By this standard, anything that acts before implantation is contraception, not abortion.
Some religious and ethical frameworks define life as beginning at fertilization, the moment egg and sperm combine. Under that definition, any intervention that could theoretically interfere with a fertilized egg’s development would be considered an abortifacient. A small number of researchers have raised the possibility that levonorgestrel could theoretically slow the transport of a fertilized egg through the fallopian tube, potentially causing it to miss its window for implantation. However, the FDA reviewed the totality of evidence and found no support for a direct effect on post-ovulatory processes.
So whether Plan B could ever be considered “abortion” depends almost entirely on which definition of pregnancy you use. By the medical and legal standard used in the United States, it is not.
How Effective Plan B Is
Plan B works best the sooner you take it. In clinical studies, the pregnancy rate was 0.8% when taken within 72 hours, translating to an effectiveness rate of 87% to 90%. Between 72 and 120 hours after unprotected sex, effectiveness drops to 72% to 87%, with a pregnancy rate of about 1.8%. It’s still worth taking in that later window, but timing matters.
Body weight also affects how well it works. Women with a BMI under 25 had a pregnancy rate of 1.3%. For women with a BMI of 25 to 30 (overweight range), the rate climbed to 2.5%. For those with a BMI over 30, the pregnancy rate reached 5.8%, a significant reduction in efficacy. If you weigh more than about 175 pounds, a copper IUD inserted as emergency contraception is generally the more reliable option.
Common Side Effects
Plan B’s side effects are mild and short-lived, typically lasting only a few days. The most common ones include nausea, headache, fatigue, dizziness, breast tenderness, and lower abdominal cramps. Some women experience light spotting or a heavier-than-usual next period. Your cycle may shift by a few days to a week. If your period is more than a week late, a pregnancy test is a reasonable next step.
There are no known long-term health effects from taking Plan B, and it’s safe to use more than once. It does not affect future fertility.
The Legal Distinction
Under U.S. federal law and FDA classification, Plan B is emergency contraception, not an abortifacient. This distinction has practical consequences. Even in states with strict abortion bans, Plan B remains legal and available over the counter to anyone, with no age restriction and no prescription required. It’s stocked alongside other contraceptives in most pharmacies. The legal protections around Plan B are separate from those governing abortion medication, because regulators treat them as fundamentally different products with different mechanisms.