WHO/HRP. Researchers working with the UNDP/UNFPA/WHO/World Bank Special Programme of Research, Development and Research Training Human Reproduction have confirmed that the use of levonorgestrel alone for emergency contraception is more effective and produces adverse effects in considerably fewer users than the Yuzpe regimen (consisting of combined oral contraceptives containing an estrogen and a progestogen in two doses with an interval of 12 hours).
Levonorgestrel is one of the two active compounds in combined oral contraceptive pills. In the regimen used for emergency contraception, two pills each containing 0.75 mg of levonorgestrel are administered with an interval of 12 hours. Calculations of the number of pregnancies that could have occurred if no treatment had been given showed that levonorgestrel had prevented 85% of unintended pregnancies as compared to 57% with the Yuzpe regimen. The success rate for both treatments is even higher among women who started treatment earlier (within 24 hours) - 95% for levonorgestrel and 77% for the Yuzpe method.
Levonorgestrel was also much better tolerated by the women than the Yuzpe regimen. Adverse reactions were the same for both methods - nausea, vomiting, dizziness, fatigue, headache, breast tenderness and low abdominal pain - but they were reported far less frequently in those taking levonorgestrel. Half the women taking the Yuzpe regimen reported nausea compared to only a quarter of those taking levonorgestrel. Vomiting occurred in 20% of the women taking the Yuzpe regimen, and in only 6% of those taking levonorgestrel.
Reference: Task Force on Postovulatory Methods of Fertility Regulaton. Randomised controlled trial of levonorgestrel versus the Yuzpe regimen of combined oral contraceptives for emergency contraception. The Lancet, 8 August 1998.