The morning after pill is a form of emergency contraception used to prevent pregnancy after unprotected sexual intercourse or contraceptive failure. It is not a routine form of birth control and is intended only for emergency use. The two main types available are those containing the hormone levonorgestrel (a progestin) and the drug ulipristal acetate (a selective progesterone receptor modulator). These medications intervene quickly to prevent a pregnancy from starting.
How the Morning After Pill Works
The primary mechanism of action for both levonorgestrel and ulipristal acetate is to delay or inhibit ovulation, the release of an egg from the ovary. Preventing the egg’s release ensures no egg is available for fertilization by sperm, which can remain viable in the reproductive tract for several days.
Levonorgestrel works by suppressing the surge of Luteinizing Hormone (LH) that normally triggers ovulation. If the LH surge has already begun, levonorgestrel is generally unable to prevent the egg’s release. Ulipristal acetate, however, is a progesterone receptor modulator that can postpone follicular rupture even after the LH surge has started. This difference explains why ulipristal acetate is effective over a longer window of up to five days, while levonorgestrel is most effective within three days. Neither medication is effective if a pregnancy has already been established.
Pharmacokinetics: Drug Clearance and Half-Life
Pharmacokinetics describes how a drug moves through the body, including its absorption, distribution, metabolism, and excretion. The time it takes for the concentration of a drug to reduce by half is known as its elimination half-life. For levonorgestrel, the mean elimination half-life is approximately 24 to 28 hours.
This means the substance is typically cleared from the bloodstream within a few days, usually within 72 hours. Ulipristal acetate has a slightly longer half-life, estimated to be around 32 hours. Consequently, ulipristal acetate is fully metabolized and excreted from the system within three to five days. The active ingredients of both contraceptives are processed by the liver and excreted through urine and feces.
While the chemical compound is physically cleared relatively quickly, the medication’s hormonal effects on the reproductive system can persist. Because of this rapid clearance, the pill does not offer ongoing contraception. Once the drug is out of the system, a person is immediately susceptible to pregnancy from subsequent unprotected intercourse.
Immediate Physical Side Effects
Users commonly experience transient physical symptoms due to the high dose of hormones while the drug is being cleared. Nausea and abdominal pain are frequently reported side effects. Individuals may also experience vomiting, headache, dizziness, fatigue, and temporary breast tenderness.
These immediate side effects are usually mild and short-lived, generally resolving within 24 hours of taking the pill. If vomiting occurs within two to three hours, consult a healthcare professional, as a repeat dose might be necessary if the drug was not fully absorbed. Taking the pill with food may help mitigate gastrointestinal discomfort.
Lingering Hormonal Effects on the Menstrual Cycle
Even after the active drug has left the body, the substantial hormonal dose can temporarily disrupt the body’s natural cycle timing. The pill’s action of delaying ovulation can alter the expected start date of the next period, which commonly arrives either earlier or later than anticipated, typically by up to a week.
The flow of the next period may also be affected, sometimes being heavier or lighter than usual. Some individuals experience unscheduled bleeding or spotting in the weeks following ingestion due to the temporary hormonal imbalance. This breakthrough bleeding is generally light and short-lived.
If the period is delayed by more than seven days, take a pregnancy test to rule out contraceptive failure. For most people, the menstrual cycle should return to its normal rhythm within one to two cycles. The hormonal impact is temporary, and there are no long-term effects on fertility.