Pharmacology of Emergency Contraception
Emergency contraception pills (ECPs), often called the "morning-after pill," are short-term, high-dose hormonal medications taken after unprotected sex to prevent an unintended pregnancy. There are two main types of ECPs available: levonorgestrel-based pills and ulipristal acetate-based pills. Both work primarily by delaying or inhibiting ovulation, preventing the release of an egg from the ovary. They do not terminate an existing pregnancy. The duration the pill remains in the body and its peak effectiveness are determined by its specific active ingredient and the body's metabolic processes.
Levonorgestrel-based pills
Levonorgestrel is a synthetic progestin hormone that mimics the body's natural progesterone. It is the active ingredient in over-the-counter ECPs such as Plan B One-Step and its generic versions. When taken, levonorgestrel is quickly absorbed into the bloodstream.
- Absorption and Metabolism: After oral administration, it reaches its maximum concentration in the blood within 1.6 hours. The liver is primarily responsible for metabolizing the hormone using the CYP3A4 isoenzyme.
- Half-Life: The average half-life, or the time it takes for half the medication to be eliminated from the bloodstream, is about 27.5 hours, though this can vary from 24 to 32 hours depending on the individual.
- Clearance: It typically takes about five half-lives for a drug to be almost fully cleared from the body, meaning levonorgestrel is usually gone within 5 to 6 days. The liver metabolizes it, and its byproducts are excreted through both urine (approximately 45%) and feces (approximately 32%).
- Effective Window: While it may linger in the body for days, its effectiveness is highest when taken as soon as possible, and diminishes significantly after 72 hours (3 days). It provides no continued protection against subsequent acts of unprotected sex.
Ulipristal Acetate-based pills
Ulipristal acetate (UPA), sold under the brand name Ella, is a selective progesterone receptor modulator (SPRM) that requires a prescription. It is a more effective option, especially when taken later in the post-coital window.
- Absorption and Metabolism: UPA is rapidly absorbed, reaching maximum concentration in approximately one hour. It is also primarily metabolized by the CYP3A4 enzyme in the liver. Its active metabolite, mono-demethyl-ulipristal acetate, also contributes to its effects.
- Half-Life: The plasma half-life of UPA is about 32.4 hours, which is longer than levonorgestrel.
- Clearance: Similar to levonorgestrel, it takes several days for UPA to be fully cleared from the system.
- Effective Window: UPA remains highly effective for up to 120 hours (5 days) after unprotected sex. It is considered more effective than levonorgestrel, particularly in the later part of the 3- to 5-day window. Because of its different mechanism, it's crucial to wait 5 days before starting or restarting regular hormonal birth control.
Factors Affecting Duration and Effectiveness
Several physiological and external factors can influence how long an ECP remains in the body and how effectively it works.
- Body Weight and BMI: Research shows that levonorgestrel-based ECPs may be less effective in individuals weighing more than 165 pounds or with a BMI over 25, possibly due to altered metabolism. Ulipristal acetate (Ella) is more effective in heavier individuals but also has a weight limitation, with reduced efficacy starting around 195 pounds.
- Drug Interactions: Certain medications and herbal supplements can accelerate the metabolism of ECPs in the liver, reducing their effectiveness and time in the body. This includes some anti-seizure medications, St. John's Wort, certain antibiotics, and HIV medication. Always consult a healthcare provider about potential interactions.
- Timing of Dose: The most critical factor for both types of ECPs is how quickly they are taken after unprotected sex. While the pill's metabolites linger in the system for several days, its pregnancy-preventing effect is immediate and works by delaying ovulation. Taking it sooner maximizes the chance of preventing ovulation before it can occur.
- Individual Metabolism: Genetic variations, age, and the health of one's liver and kidneys can cause individual differences in metabolic speed, affecting clearance time.
Comparing Levonorgestrel and Ulipristal Acetate
| Feature | Levonorgestrel (Plan B) | Ulipristal Acetate (Ella) |
|---|---|---|
| Availability | Over-the-counter at most pharmacies. | Prescription only. |
| Time Window | Most effective within 72 hours (3 days) of unprotected sex. | Effective up to 120 hours (5 days) after unprotected sex. |
| Half-Life | Approximately 27.5 hours on average. | Approximately 32.4 hours. |
| Mechanism | Prevents or delays ovulation; may thicken cervical mucus. | Prevents or delays ovulation by modulating progesterone receptors. |
| Weight Impact | Less effective for those over 165 pounds. | Less effective for those over 195 pounds. |
| Ongoing Contraception | No specific waiting period before starting hormonal contraception. | Must wait at least 5 days before starting hormonal contraception. |
| Effect on Period | Can cause changes to the next period. | Can cause a delayed period. |
Conclusion
Understanding how long does an emergency pill last in a body involves recognizing the difference between the drug's lingering presence and its window of contraceptive effectiveness. While metabolites of ECPs can remain in the system for up to a week, the pill's primary function of delaying ovulation occurs almost immediately and provides no lasting protection. For this reason, the sooner an ECP is taken after unprotected sex, the higher its efficacy. The specific active ingredient (levonorgestrel vs. ulipristal acetate), individual factors like body weight and metabolism, and other concurrent medications all play a role in the precise duration and efficacy. Consulting a healthcare provider is recommended to determine the best emergency contraceptive option and to address any concerns regarding side effects or future contraceptive plans.
