Plan B starts working the same day you take it. Even so, you can still get pregnant the very next day after taking Plan B if you have unprotected sex again.
Plan B One Step only helps prevent a pregnancy that could result from sex that happened in the 5 days before you took it. It does not protect against pregnancy from sex that happens afterward.
You may notice some side effects for a few days or even up to a week after taking it due to its hormonal effects. These can include light irregular bleeding, nausea, abdominal pain, fatigue, headaches, breast tenderness, dizziness, and vomiting.
After taking Plan B, your period may also come earlier or later than expected. Because emergency contraception does not have a lasting contraceptive effect, you can still become pregnant if you have unprotected sex the day after taking it.
For that reason, after using Plan B, it is recommended to start using another birth control method right away, such as condoms, birth control pills, contraceptive injections (Depo Provera), or an intrauterine device (IUD). Plan B works mainly in two ways: it delays ovulation, and if ovulation has already happened, it helps prevent the sperm and egg from meeting.
If you would like more information about how long Plan B takes to work or about other birth control options, the best option is to see a gynecologist or your primary care provider.