What will happen if you take multiple birth control pills directly after intercourse?

Prevent Pregnancy: First of all, you may have side effects from multiple pills. The most common ones are nausea or vomiting which goes away quickly. Second, the pills will likely prevent a pregnancy, that might have otherwise grown in the uterus, from doing that. The pills alter the uterus to make it temporarily difficult for a potential pregnancy to stick and grow there.

Emergency contracept: This is one of the options for emergency contraception, taking high-dose estrogen/progesterone to prevent pregnancy after unprotected intercourse. If taken properly (# of pills depends on the brand of birth control pill), it can reduce chance of pregnancy by~75-80%. A common side effect is nausea +/- vomiting.