Ulipristal acetate, used in emergency contraception up to 120 hours after unprotected intercourse, acts as:

• A A GnRH antagonist preventing the LH surge required for ovulation
• B A selective progesterone receptor modulator (SPRM) — at low doses it inhibits/delays ovulation; at higher doses it also affects endometrium ✓
• C A pure progesterone receptor antagonist (mifepristone-like) causing endometrial shedding
• D An estrogen receptor agonist suppressing FSH and LH by negative feedback

Explanation

Ulipristal acetate (UPA) is a selective progesterone receptor modulator with predominantly antagonistic activity at high doses and mixed agonist-antagonist activity at lower doses. Its primary mechanism in emergency contraception is inhibition or delay of ovulation, which it can achieve even after the start of the LH surge — giving it an advantage over levonorgestrel (which loses efficacy post-surge). It has limited post-fertilisation effects at approved EC doses.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.