How long would it take until the birth control pill is effective?

Possible Immediate: Immediate if started to take the first day of your period. After that the risk of geting pregnant is higher. If sunday start or a couple of days after your period you need added protection that whole month.

Effectiveness of BCP: It takes about 2 weeks for a bcp to be effective if you've started soon after your menses. Nevertheless, i advise my patients to use condoms the first month, and to rely on the bcp the second months.