Took one flagyl oral pill and taking metronidazole currently. can this delay my period?

Are you a guy?: Flagyl is a brand name for metronidazole, so you are (or she is) taking a single antibiotic. Theoretically, metronidazole & other antibiotics can reduce the effectiveness of oral contraceptive, altho the chance from this specific antibiotic is very small. If your (or her) period is late, I recommend a pregnancy test. Best wishes.