From The Population Institute of Canada
Two dedicated emergency contraceptive products are available: Preven and Plan B. Both work by delaying or inhibiting ovulation, inhibiting fertilization or inhibiting implantation of a fertilized egg. Preven and Plan B are little more than high-doses of normal birth control pills. Plan B is a progestin-only pill, while Preven is a progestin-estrogen combination. In California, Alaska, and Washington State, some pharmacies will provide emergency contraceptive pills without requiring you to see anyone except the pharmacist. In addition, emergency contraception is available over the counter in Quebec. If Preven or Plan B are not available, women can get the same results by taking several active birth control pills at once (see http://ec.princeton.edu/worldwide/default.asp for protocols). In addition to hormone-base methods of emergency contraception, the insertion of an IUD within 5 days of having sex, can also work.