Can oral pill of metronidazole interfere with birth control or cause vaginal spotting? thank you?

Metronida. likely ok: The current thinking is that except for rifampin, other commonly-used antibiotics (like metronidazole and many others) do not alter the effectiveness of birth control pills. This is likely presuming a person is not sick with vomiting or rapid diarrhea, or some other condition that decreases the absorption of a swallowed pill. One can look up: http://www.ncbi.nlm.nih.gov/pubmed/12063491