The phrase “yesterday’s pill” commonly refers to Emergency Contraception (EC), a form of backup birth control used after unprotected sexual intercourse or contraceptive failure. EC is a time-sensitive medication designed to prevent an unintended pregnancy from starting. It is not intended for routine use, and taking the pill as quickly as possible after the event is paramount to its function.
Identifying Emergency Contraception and Its Mechanism of Action
Emergency contraception pills contain high doses of hormones that work to prevent pregnancy. The two primary types of pills available are those containing levonorgestrel, such as Plan B One-Step and its generics, and ulipristal acetate, marketed as Ella. Levonorgestrel is a synthetic progestin, a hormone similar to one naturally produced in the body, while ulipristal acetate is a selective progesterone receptor modulator.
The main scientific mechanism for both pill types is the inhibition or delay of ovulation, which is the release of an egg from the ovary. By postponing this release, the pill ensures that no egg is available to be fertilized by sperm that may be present in the reproductive tract. Since sperm can survive for several days, delaying ovulation until the sperm have died prevents conception.
Ulipristal acetate is considered more effective at delaying ovulation, especially if taken closer to the time the egg would normally be released. It can work even after the body has begun the surge of luteinizing hormone (LH), which triggers ovulation. Hormonal emergency contraception works by preventing a pregnancy from being established; it does not affect a fertilized egg that has already implanted in the uterus.
Critical Timing for Maximum Efficacy
The effectiveness of emergency contraception pills is highly dependent on how quickly they are taken after unprotected sex. Both types are most effective when ingested immediately. The primary difference between the two main types lies in their effective window.
Levonorgestrel-based pills, like Plan B, are most effective when taken within 72 hours of unprotected intercourse. While they may offer some reduced effectiveness up to 120 hours, taking them in the first 24 hours provides the highest chance of success. Ulipristal acetate, or Ella, maintains a consistent level of effectiveness for the full 120 hours after unprotected sex.
Because ulipristal acetate can interrupt the ovulation process even later in the cycle, it is the preferred option for those nearing the end of the five-day window. The efficacy of all emergency contraception decreases rapidly as the time between unprotected sex and ingestion increases, so the goal is always to take the pill as soon as possible.
Access, Cost, and Common Side Effects
The accessibility of emergency contraception varies depending on the specific drug. Levonorgestrel-only pills are available over-the-counter at pharmacies, clinics, and some retail stores without a prescription or age restriction. Ulipristal acetate, however, is a prescription-only medication, requiring a consultation with a healthcare provider or pharmacist.
The cost for levonorgestrel pills can range widely, typically from about $10 for generic versions to $50 for brand names, though insurance plans often cover the cost. Ulipristal acetate tends to be more expensive without insurance, but it is also frequently covered by health plans. Many community health centers and family planning clinics offer both types of emergency contraception at a reduced cost or for free to those who qualify.
After taking an emergency contraceptive pill, a person may experience temporary and mild side effects. These can include nausea, headache, dizziness, and abdominal pain. Changes to the next menstrual period are also common, with the period potentially arriving a few days earlier or later than expected, or being heavier or lighter than usual.
Distinguishing Emergency Contraception from the Abortion Pill
A major point of public confusion surrounds the difference between emergency contraception and the “abortion pill.” The fundamental distinction lies in when each medication is used in relation to the start of a pregnancy. Emergency contraception pills are strictly preventative measures taken before a pregnancy has been established in the uterus.
The medical definition of pregnancy begins when a fertilized egg implants into the uterine lining, which takes several days after fertilization. Emergency contraception works by disrupting the reproductive process, primarily ovulation, before this implantation can occur. Conversely, the “abortion pill” (medication abortion), which typically involves the drugs mifepristone and misoprostol, is used to terminate a pregnancy that has already been confirmed and established in the uterus.
Mifepristone blocks the hormone progesterone, which is necessary to sustain a pregnancy, while misoprostol causes the uterus to contract. Emergency contraception cannot cause an abortion and will not harm a pregnancy if a person is already pregnant when they take the pill.