For Plan B and its generics – it is a progesterone. Progesterone is the hormone of pregnancy. So it tells the body that you are pregnant and thus prevents the body from releasing an egg if it is about to release an egg. And some studies have shown that it blocks the sperm capacitation (getting into the egg). For Ella or ulipristal acetate, it mainly works by blocking the body from releasing an egg. because all of these are one time doses, there are no long term effects. We don’t recommend using these methods as your primary method of birth control because it is a “strong jolt” or “reset” to the normal flow but better this than pregnant. Also, you can use emergency contraception multiple times. We also don’t recommend using these methods b/c they don’t work if you have released your egg or there is lower efficacy. So better to prevent the egg from being released in the first place and that is done with the IUD with hormone, the implant, the shot, the ring, the patch, the pill.