The original treatment schedule for progestin-only emergency contraceptive pills was one 0.75 mg dose within 72 hours after unprotected intercourse, and a second 0.75 mg dose 12 hours after the first dose. However, recent studies1, 2 have shown that a single dose of 1.5 mg is as effective as and causes no more side effects than two 0.75 mg doses 12 hours apart. (Another study3 found that two 0.75 mg doses 24 hours apart were just as effective as two 0.75 mg doses 12 hours apart.)
1. von Hertzen H, Piaggio G, Ding J, Chen J, Song S, Bártfai G, Ng E, Gemzell-Danielsson K, Oyunbileg A, Wu S, Cheng W, Lüdicke F, Pretnar-Darovec A, Kirkman R, Mittal S, Khomassuridze A, Apter D, Peregoudov A. Low dose mifepristone and two regimens of levonorgestrel for emergency contraception: a WHO multicentre randomized trial. Lancet 2002;360:1803-1810.
2. Arowojolu AO, Okewole IA, Adekunle AO. Comparative evaluation of the effectiveness and safety of two regimens of levonorgestrel for emergency contraception in Nigerians. Contraception 2002;66:269-273.
3. Ngai SW, Fan S, Li S, Cheng L, Ding J, Jing X, Ng EHY, Ho PC. A randomized trial to compare 24h versus 12h double dose regimen of levonorgestrel for emergency contraception. Human Reproduction 2004;20:307-311.