Plan B may not work well during ovulation. It works mainly by delaying or stopping ovulation, so it is most useful when taken before the egg is released. If ovulation has already happened, Plan B may have little or no meaningful effect.
This means timing matters. Plan B is most effective when taken as soon as possible after unprotected sex, especially before the LH surge, which is the hormone change that triggers ovulation. Its effect becomes much less reliable as ovulation gets closer.
If sex happened during the fertile window or ovulation may have already occurred, other emergency contraception options may work better. The copper IUD is the most effective option and can be used within 5 days after unprotected sex. Ulipristal acetate may also work better than Plan B close to ovulation, but it may not work once ovulation has already occurred.
A pharmacist, OB-GYN, or primary care clinician can help choose the best option based on timing, medications, and pregnancy risk. Avoid taking extra doses of Plan B unless a healthcare professional recommends it.