How long after starting the pill for the first time does it take for it to become 99% effective in your body?

It depends on whether you’re taking combination pills or progestin-only pills.

You may start the combination pill at any time. If you start within five days after the start of your period, you are protected against pregnancy right away. You will not need to use a backup method of birth control. That means that if your period starts on a Wednesday morning, you can start the pill up to Monday morning to be protected right away. If you start at any other time during your menstrual cycle, you will be protected from pregnancy after seven days.

Use another method of birth control — like a condomfemale condomdiaphragm, or sponge — if you have vaginal intercourse during the first week of use.

You may start the progestin-only pill at any time. Use another method of birth control if you have vaginal intercourse during the first 48 hours of progestin-pill use — protection will begin after two days.

Learn more about the pill.