You can start taking birth control pills as soon as you get them — any day of the week, and anytime during your menstrual cycle. But when you’ll be protected from pregnancy depends on when you start and the kind of pill you’re using. You may need to use a backup birth control method (like condoms) for up to 7 days.
There are 2 types of birth control pills (combination pills and progestin-only pills). Combination pills have the hormones estrogen and progestin — these are the most common types of birth control pills. If you start taking combination pills within 5 days after your period starts, you’ll be protected from pregnancy right away. If you start combination pills any other time, you need to take the pill for 7 days before you’re protected from pregnancy. Use another method of birth control — like a condom — if you have penis-in-vagina sex during your first week on the pill.
If you take progestin-only pills, you’ll be protected from pregnancy after 48 hours (2 days) on the pill. If you have penis-in-vagina sex during those first 2 days, use another method of birth control, like a condom. You must take progestin-only pills at the same time every day. If you take your pill more than 3 hours past your usual time, use a backup method of birth control for the next 48 hours (2 days).
So there’s no need to wait for the first day of your period to start taking your birth control pills — you can start whenever you like! At the longest, you’ll only have to wait seven days for the pills to start being effective.