I was off birth control pills for a month. in the first week of starting my pills again i had sex will my bc be effective or should i take plan b?

It depends…: In general, most BCPs are effective immediately when started as designed right after the period. In that circumstance, Plan B would be unnecessary. This would not be true when taking certain extremely low dose BCPs and when the BCP was not started at the beginning of the cycle.

Uncertain: I don't know which oral contraceptive you are taking. You can check on-line to see how long it takes your particular birth control to be effective. Or your pharmacist can advise you. Depending on this info, you can make the decision as to whether or not, it would be productive to take Plan B One Step.