So the pill is effective after one week after starting to take it for the first time, or each month?

Great question!

When someone starts a new hormonal method of birth control (like the pill) they need to use a back up method (like condoms or practicing abstinence) for one week before that method will be effective. After that first week, the hormones will be integrated into the body and can help prevent pregnancy up to 92-99% when taken correctly, but it still a good idea to use condoms to protect against STIs. But keep in mind that your birth control is only going to be effective if you’re taking it correctly.  If you are on the pill, you need to remember to take it at the same time, every day.