Morning after pill
Plan B

It works by releasing a chemical that keeps a woman's eggs from attaching to the uterine wall.(making her start her period) It can be taken up to 72 hours after unprotected sex and is easily gotten from planned parenthood clinics or your regular doctor for about 20$ a pill. One is all you need, however some pharmacies will not stock it so your doctor may have to change the scrip to birth control form. If this happens you take 3 or 4 pills at a time and it works the same. The names for it are morning after pill, or plan b you can order it from those internet pharmacies too.