Plan B pills are effective at preventing pregnancy but not guaranteed to work in every case. They reduce the risk of pregnancy by approximately 60–90% when taken within 72 hours of unprotected intercourse, with higher effectiveness the sooner they're taken. However, their effectiveness decreases with time, and they work less reliably in people with higher body weights.
Plan B (levonorgestrel) works primarily by delaying or preventing ovulation. It does not terminate an existing pregnancy and cannot prevent pregnancy if ovulation has already occurred. This timing variability means its effectiveness varies considerably depending on where someone is in their menstrual cycle when taken. It's also less effective or ineffective if taken after ovulation has begun.
Other factors affecting reliability include individual metabolism, drug interactions with certain medications, and proper timing of administration. Some women may vomit the pill shortly after taking it, reducing effectiveness. Additionally, Plan B is less reliable than other emergency contraceptive options like copper IUDs or prescription medications such as ulipristal acetate (ella), which can be effective for up to 5 days.
Plan B should be considered a backup option rather than a primary contraceptive method. People concerned about contraceptive reliability should discuss long-term options with a healthcare provider. If a period doesn't arrive when expected after taking Plan B, pregnancy testing is recommended.