1.5mg of levonorgestrel (taken within 72 hours) prevents at least half the pregnancies that occur following 4-6% of unprotected sexual intercourse, by blocking and/or delaying ovulation.
A Scottish study of emergency contraceptive pills found increasingly high failure rates in the five days prior to ovulation; this was also associated with obesity. Levonorgestrel failed 50% more often when the woman’s Body Mass Index (BMI) was above 25m/kg2, and failed three times more often when the BMI was above 30m/kg2. In fact, this failure rate suggests that levonorgestrel is ineffective for women greater than 70kg.
The study made no comment about the efficacy of general contraceptive products containing levonorgestrel, and observed that insertion of a copper intrauterine device (IUD) following unprotected sexual intercourse is more reliable than hormone therapy.
Please review full product information before prescribing.
- Glasier A, Cameron ST, Blithe D, Scherrer B, Mathe H, Levy D, et al. Can we identify women at risk of pregnancy despite using emergency contraception? Data from randomized trials of ulipristal acetate and levonorgestrel. Contraception 2011; 84(4): 363-7.