This is a great question. The way Plan B works is by stopping ovulation. There used to be some confusion about whether it could work after ovulation has begun, but research now shows that it does not (and the FDA made this permanent change to the package label in 2022). So to answer your question, no. It will not work if ovulation has started.

Unless you’re 100% sure of when you’ve ovulated, it’s still worth taking Plan B. Or, if you have access to a medical provider, consider having an IUD placed ASAP. An IUD (intrauterine device) is super effective at preventing pregnancy if placed up to 5 days after unprotected sex.