The morning-after pill is a form of emergency contraception used to reduce the risk of pregnancy after unprotected sex or contraceptive failure. It works primarily by delaying or preventing ovulation, which is the release of an egg from the ovaries. Without ovulation, fertilization cannot occur, so pregnancy is prevented
. There are two main types of morning-after pills:
- Levonorgestrel-based pills (e.g., Plan B One-Step) are most effective when taken within 72 hours (3 days) after unprotected sex but can be used up to 5 days. Their effectiveness is highest if taken as soon as possible, with about 95% effectiveness within 24 hours, decreasing to about 58% at 72 hours
- Ulipristal acetate-based pills (e.g., ellaOne) can be taken up to 120 hours (5 days) after unprotected sex and maintain higher effectiveness longer than levonorgestrel pills. They work by blocking progesterone, which is necessary for ovulation and maintaining early pregnancy
The morning-after pill does not terminate an existing pregnancy and will not work if ovulation has already occurred and fertilization has taken place
. It is not an abortion pill. For best results, the pill should be taken as soon as possible after unprotected sex. If vomiting occurs within 2-3 hours of taking the pill, another dose may be needed because absorption could be affected
. Common side effects are mild and short-term, including nausea, headache, dizziness, abdominal pain, and changes in menstrual bleeding
. In summary, the morning-after pill prevents pregnancy mainly by delaying ovulation, is most effective when taken promptly after unprotected sex, and does not affect an established pregnancy
