There are two types of emergency contraceptives:
Emergency contraceptive pills (ECPs) Intrauterine device (IUD) Emergency contraceptive pills (ECPs)
With ECPs, higher doses of the same hormones found in regular birth control pills prevent pregnancy by keeping the egg from leaving the ovary or keeping the sperm from joining the egg. While it is possible that ECPs might work by keeping a fertilized egg from attaching to the uterus, the most up-to-date research suggests that ECPs do not work in this way. In the United States, there is only one FDA-approved pill that is specially made to be used as an ECP. It is called Plan B. However, when used in a certain way, some regular birth control pills also can be used as ECPs.
Plan B — Plan B is a progestin-only ECP. It is made for use as emergency contraception. Plan B is like progestin-only birth control pills, but contains higher levels of the hormone. The instructions for Plan B say to take the two pills 12 hours apart. But research has shown that taking both pills at the same time works just as well and does not increase side effects. Higher dose of regular birth control pills — The number of pills in a dose is different for each pill brand, and not all brands can be used for emergency contraception. The pills are taken in 2 doses (1 dose right away, and the next dose 12 hours later). Always use the same brand for both doses, and be sure to use the active pills, not the reminder pills.
You should always take ECPs as soon as you can after having unprotected sex, but they can work up to 5 days later. Women who are breastfeeding or cannot take estrogen should use progestin-only ECPs (like Plan B). Some women feel sick and throw up after taking ECPs. If you throw up after taking ECPs, call your doctor or pharmacist.
Intrauterine device (IUD)
The IUD is a small, T-shaped device placed into the uterus by a doctor within 5 days after having unprotected sex. The IUD works by keeping the sperm from joining the egg or keeping a fertilized egg from attaching to the uterus. Your doctor can remove the IUD after your next period. Or, it can be left in place for up to 10 years to use as your regular birth control method.
This answer is based on source information from Office of Women's Health.