Effects of Ulipristal Acetate
Ulipristal acetate is a highly effective emergency contraceptive that works primarily by delaying or inhibiting ovulation, with potential effects on the endometrium, and is more effective than levonorgestrel when taken 3-5 days after unprotected intercourse. 1, 2
Primary Mechanism and Effectiveness
- Ulipristal acetate (30 mg) is a selective progesterone receptor modulator that acts as an emergency contraceptive by inhibiting or delaying ovulation when taken within 5 days (120 hours) of unprotected sexual intercourse 1
- It delays follicular rupture by postponing the luteinizing hormone (LH) surge or preventing follicular rupture even after LH surge has started 2
- Ulipristal acetate is effective throughout the entire 120-hour window after unprotected intercourse, unlike levonorgestrel which shows decreased effectiveness after 72 hours 1, 3
- Studies show ulipristal acetate has similar effectiveness to levonorgestrel when taken within 3 days but is more effective than levonorgestrel when taken 3-5 days after unprotected intercourse 1, 4
Comparative Effectiveness
- A meta-analysis demonstrated that ulipristal acetate has a pregnancy risk 42% lower than levonorgestrel up to 72 hours and 65% lower in the first 24 hours following unprotected intercourse 3, 4
- Ulipristal acetate may be more effective than levonorgestrel in women who are overweight or obese 1
- When compared to other emergency contraception options, copper IUDs are most effective, followed by ulipristal acetate, then levonorgestrel, with combined estrogen-progestin regimens being least effective 1
Timing and Administration
- Ulipristal acetate should be taken as soon as possible within 5 days of unprotected intercourse 1
- Unlike levonorgestrel, ulipristal acetate maintains consistent effectiveness throughout the entire 5-day window after unprotected intercourse 5, 6
- After using ulipristal acetate, any regular contraceptive method can be started immediately, but barrier contraception or abstinence is needed for 14 days or until the next menstrual period, whichever comes first 1
- Advance provision of emergency contraception may be provided to ensure availability when needed 1
Side Effects and Safety
- The most common adverse reactions (≥10%) with ulipristal acetate are headache (18-19%), nausea (12-13%), and abdominal/upper abdominal pain (8-15%) 7
- Other common side effects include dysmenorrhea (7-13%), fatigue (6%), and dizziness (5%) 7
- Onset of menses is typically delayed by 2-3 days following treatment 2
- The safety profile in adolescents aged 17 and younger is similar to that in adults 7
- Post-marketing reports include skin reactions such as acne, hypersensitivity reactions including rash, urticaria, pruritus, and angioedema 7
Special Considerations
- If a woman starts hormonal contraception after using ulipristal acetate, she should abstain from sexual intercourse or use barrier contraception for 14 days or until her next menses 1
- A pregnancy test is recommended if a woman does not have a withdrawal bleed within 3 weeks after taking ulipristal acetate 1
- Ulipristal acetate may interfere with the effectiveness of regular hormonal contraceptives and vice versa 7
- When combined oral contraceptives are started 2 days after ulipristal acetate intake, the ability of ulipristal acetate to delay ovulation may be reduced 7
Pharmacological Properties
- Ulipristal acetate is a selective progesterone receptor modulator with both antagonistic and partial agonistic effects at the progesterone receptor 7
- The drug's pharmacodynamic effects depend on the timing of administration in the menstrual cycle 7
- When administered in the mid-follicular phase, it inhibits folliculogenesis and reduces estradiol concentration 7
- When administered in the early luteal phase, it doesn't significantly delay endometrial maturation but decreases endometrial thickness 7