Plan B is a single-dose emergency contraception designed to reduce the chance of pregnancy after unprotected intercourse or contraceptive failure. Its active ingredient, levonorgestrel, is a synthetic hormone that temporarily alters the reproductive cycle. Understanding how long the drug remains in the body requires separating the physical presence of the chemical from its window of effectiveness. This article clarifies the distinction between the drug’s chemical clearance and its biological impact.
How the Medication Prevents Pregnancy
Plan B works by delivering a high dose of the progestin levonorgestrel, which interferes with the reproductive process. This hormonal surge primarily inhibits or delays the release of an egg from the ovary (ovulation), preventing fertilization. The pill’s effectiveness is tied to the timing of the woman’s cycle. If ovulation has already occurred, the drug is generally ineffective. Levonorgestrel prevents the beginning of a pregnancy and has no effect on a fertilized egg that has already implanted.
The Pharmacokinetics of Levonorgestrel Clearance
The question of how long Plan B “stays in your system” is answered through pharmacokinetics, which tracks the drug’s journey through the body. Levonorgestrel is metabolized in the liver and eliminated primarily through urine and feces, with approximately 45% excreted in urine. The rate of concentration decrease is measured by its elimination half-life. For levonorgestrel, the mean elimination half-life is approximately 24 to 32 hours, meaning half of the drug’s concentration is removed within this timeframe. After roughly five to seven days (about five half-lives), the drug’s active concentration in the plasma drops to a negligible level.
Understanding the Window of Effectiveness
The drug’s chemical clearance is distinct from its window of effectiveness in preventing pregnancy. Maximum effectiveness is tied to how quickly it is taken after unprotected sex. Levonorgestrel is most effective when taken within 72 hours (three days) of intercourse. Effectiveness decreases significantly with each passing day, so taking the pill as soon as possible is recommended. Once the drug is metabolized and cleared, it offers no sustained protection against future acts of intercourse.
The Return of the Menstrual Cycle
Following the high hormonal dose from Plan B, the body must adjust, which often impacts the timing of the next menstrual cycle. This hormonal shift can temporarily disrupt the cycle, causing the next period to arrive slightly earlier or later than expected. Most women will get their period within a week of the anticipated time. If the period is delayed by more than one week past the expected date, a pregnancy test should be taken. The medication’s effect is transient, and menstrual patterns typically return to their normal rhythm in the following cycle.