Oral contraceptive pills are over 99% effective at preventing pregnancy when used correctly. The main reason for failure is forgetting to take the pill every day. Skipping two or more doses of the pill will significantly decrease its effectiveness. However, if used properly the pill maintains its high level of protection and does not become less effective over time.

The pill works primarily by elevating blood levels of hormones that suppress the release of eggs from the ovary. If the pill is not absorbed from the intestinal tract because of nausea or diarrhea, it may not work. Also, some drugs, including certain anti-seizure medicines, can speed up the metabolism of the pill and render it less effective. Any women using more than one medicine should discuss potential drug interactions with her doctor.

Women who need long-term contraception should not be discouraged from using the pill. In fact, women who use oral contraceptives for at least three years will actually benefit from a decrease in risk of ovarian and uterine cancer.