Ulipristal Acetate Is More Likely Than Levonorgestrel to Prevent Ovulation

Emergency contraception (EC) pills containing the progestogen levonorgestrel (e.g., Plan B) were recently made available in the U.S. without prescription. To compare oral EC regimens' ability to prevent ovulation, investigators pooled data from three randomized trials (one of which was manufacturer sponsored). Daily blood draws and transvaginal ultrasound were used to assess hormone levels and rates of follicular rupture within 5 days of treatment with placebo, ulipristal (a selective progesterone receptor modulator), or levonorgestrel.

Overall, ulipristal acetate was four times more likely than levonorgestrel to prevent ovulation (unruptured dominant follicles in 59% vs. 15% of cycles). Although the two types of pills were equally effective…