How does Ella (ulipristal acetate) differ from Plan B (levonorgestrel) in terms of mechanism of action?

Differences Between Ella (Ulipristal Acetate) and Plan B (Levonorgestrel)

Ella (ulipristal acetate) is more effective than Plan B (levonorgestrel) for emergency contraception, especially when taken 72-120 hours after unprotected intercourse and for women who are overweight or obese. 1

Mechanism of Action

Ella (Ulipristal Acetate)

• Acts as a selective progesterone receptor modulator 2
• Primary mechanism: Delays or inhibits ovulation even when taken shortly before ovulation 3
• Has a direct inhibitory effect on follicular rupture 3
• Effective on larger follicles (up to 18mm in diameter) 4
• Works even when LH levels have begun to rise (advanced follicular phase) 2

Plan B (Levonorgestrel)

• Acts as a progestin-only emergency contraceptive 5
• Primary mechanism: Delays or inhibits ovulation 3
• Only effective if taken before the LH surge begins 3
• Only effective on smaller follicles (up to 14mm in diameter) 4
• Ineffective when taken close to ovulation 3

Efficacy Comparison

• Ulipristal acetate prevents approximately 85% of expected pregnancies 1
• Levonorgestrel prevents approximately 75-80% of expected pregnancies 1
• Meta-analysis shows ulipristal acetate has:
  • 42% lower pregnancy risk than levonorgestrel up to 72 hours after intercourse 5
  • 65% lower pregnancy risk in the first 24 hours after intercourse 5

Timing of Administration

• Ella (Ulipristal Acetate):

  • Effective up to 120 hours (5 days) after unprotected intercourse 1, 5
  • Maintains effectiveness throughout the 5-day window 6
  • More effective than levonorgestrel between days 3-5 1

• Plan B (Levonorgestrel):

  • FDA-approved for use up to 72 hours (3 days) after unprotected intercourse 5
  • Most effective when taken as soon as possible after intercourse 1
  • Efficacy decreases significantly after 72 hours 6

Special Considerations

Weight/BMI Impact

• Ulipristal acetate is more effective than levonorgestrel for women who are overweight or obese 1

Dosing

• Ella: Single 30mg dose 5
• Plan B: Single 1.5mg dose (Plan B One-Step) or two 0.75mg pills taken 12 hours apart (Next Choice) 5

Availability

• Ella requires a prescription in the United States 5
• Plan B is available over-the-counter without age restrictions 7

Clinical Application Algorithm

1. Determine time since unprotected intercourse:

   • If within 72 hours: Either method can be used, but ulipristal acetate is more effective
   • If between 72-120 hours: Ulipristal acetate is the only effective oral option

2. Consider patient weight/BMI:

   • For patients with higher BMI: Ulipristal acetate is preferred

3. Assess proximity to ovulation:

   • If close to ovulation: Ulipristal acetate is significantly more effective

4. Check accessibility:

   • If prescription access is limited and immediate treatment needed: Plan B may be more accessible

Common Pitfalls to Avoid

• Timing misconception: Don't assume both medications have the same window of effectiveness. Ella works up to 5 days, while Plan B is most effective within 3 days.

• Efficacy overestimation: Neither method is 100% effective. Ulipristal acetate prevents about 85% of expected pregnancies, while levonorgestrel prevents about 75-80%.

• Mechanism confusion: Both primarily work by delaying ovulation, not by preventing implantation of a fertilized egg.

• Weight considerations: Both medications may be less effective in women with higher BMI, but ulipristal acetate maintains better efficacy in this population.

• Repeated use: Neither medication should be used as regular contraception; they are designed for emergency use only.