Can I take metronidazole 500 mg pills while on my period? Or should I wait until period ends? Can I have sex while taking this? It's for BV

Continue treatment. You should continue treatment while on your period. However, you shouldn't have sex for the next couple of weeks -- not because of metronidazole, but sex is associated with BV and might make treatment less effective. If sex is inevitable, for sure use condoms. I hope you were tested for STDs (gonorrhea, chlamydia, trichomonas), which are much more common in women with BV.