How does Plan B (levonorgestrel) work?

How Plan B (Levonorgestrel) Works as Emergency Contraception

Plan B (levonorgestrel) primarily works by delaying or inhibiting ovulation, preventing pregnancy before fertilization occurs, and is most effective when taken as soon as possible within 72 hours after unprotected intercourse.

Mechanism of Action

Plan B contains levonorgestrel, a progestin hormone that acts through several mechanisms:

1. Primary Mechanism: Preventing Ovulation

   • Delays or inhibits the release of an egg from the ovary by preventing the luteinizing hormone (LH) surge 1
   • Most effective when taken before ovulation has begun 2
   • Cannot prevent ovulation once the LH surge has started 1

2. Secondary Effects

   • May affect sperm transport or function
   • Does NOT prevent fertilization if ovulation has already occurred 1
   • Does NOT disrupt an implanted pregnancy or harm an existing embryo 1, 2

Effectiveness and Timing

The effectiveness of Plan B is highly time-dependent:

• Reduces pregnancy risk by approximately 75-88% when used correctly 3, 4
• Timing is critical: "The sooner you take emergency contraception, the better it works" 4
• Most effective when taken immediately after unprotected intercourse
• Effectiveness decreases significantly with each passing day 3
• FDA-approved for use within 72 hours (3 days) after unprotected sex 4
• Mathematical modeling suggests taking it immediately after sex could potentially reduce pregnancy risk by up to 91%, but actual effectiveness ranges from 49-67% due to treatment delays 5

Important Clinical Considerations

• Not for regular contraception: Plan B is less effective than regular birth control methods and should only be used as emergency backup 4
• Will not affect an existing pregnancy: Not effective if already pregnant and will not harm an existing pregnancy 4
• Not an abortion pill: Works primarily by preventing ovulation, not by disrupting implantation 1, 2
• No protection against STIs: Does not protect against HIV or other sexually transmitted infections 4

Comparison with Other Emergency Contraception Methods

• Ulipristal acetate (Ella) is more effective than levonorgestrel, especially:

  • When taken 3-5 days after unprotected intercourse 6
  • For overweight or obese women 6
  • Has a longer window of effectiveness (up to 120 hours/5 days) 6, 7

• Copper IUD is the most effective emergency contraception method overall 8

  • Can be inserted up to 5 days after unprotected intercourse 3
  • Provides ongoing contraception after insertion

Common Side Effects

• Changes in menstrual cycle (heavier/lighter periods or early/late periods)
• Nausea
• Lower abdominal pain
• Fatigue
• Headache
• Dizziness
• Breast tenderness 4

Important Warnings

• Medication interactions: Effectiveness may be reduced when taken with certain medications:

  • Efavirenz (HIV medication)
  • Rifampin (tuberculosis treatment)
  • Certain anti-seizure medications 4

• Vomiting: If vomiting occurs within 2 hours of taking Plan B, contact a healthcare provider about repeating the dose 4

• Delayed period: If your period is more than a week late after taking Plan B, take a pregnancy test 4

• Severe abdominal pain: May indicate ectopic pregnancy requiring immediate medical attention 4

Plan B represents an important option for preventing unintended pregnancy after contraceptive failure or unprotected intercourse, but its effectiveness is maximized when taken as soon as possible after the event.