Levonelle and Schering PC4 morning after pills, 2000.

UNITED KINGDOM - MARCH 05: Levonelle and PC4 are both emergency contraceptives (morning after pills). PC4 is a combined emergency contraceptive pill containing both oestrogen and progesterogen, while Levonelle is a progestrogen only emergency pill. These pills may help to inhibit or delay ovulation, interfere with fertilization, or alter the endometrium (the lining of the uterus), thereby preventing implantation of a fertilised egg. They are effective only if taken within 72 hours of sexual intercourse. (Photo by SSPL/Getty Images)