Emergency contraception It is believed that the main mechanism of action of high-dose progestin emergency contraception is inhibition of ovulation, but other mechanisms may be involved Taken together, these data are highly supportive of the concept that levonorgestrel emergency contraception has little or no effect on postovulation events but is highly effective when taken before ovulation. viagra online Levonorgestrel emergency contraception does not affect implantation and is not abortifacient.