When should I take Plan B pill?

Dear Ask The Doctor: So my friend is freaking out. Last week she had sex (with a condom) with a boy and regreted, so two days later she took plan b. then a week later she had sex with a boy she is in love with and feels horrable. they used protection apparently, but after they already started. should she get plan B again? She does not want to get pregnant, she just turned 18 not too long ago.

Yes she can take Plan B again. However, Plan B pill is effective when taken within 72 hours (3 days) of unprotected sex. Effectivity is ~90-95% when taken within 24 hours (day 1) and decreases to ~55-60% when taken within 72 hours (day 3). If she took the plan B pill within 3 days then the likelihood of conceiving is small. If she had unprotected sex more than 3 days ago, then the effectivity of the plan B pill would be much less, if at all. Please advise your friend to practice safe sex because that what she really needs to do. She can ask her doctor about birth control pills, patch or depot injections. I do hope I have answered your question and take care.