Is it possible for metronidazole to be effective for bronchitis?

Possible, unlikely: Metronidazole is not recommended for this purpose. If you're on metronidazole for something else (certain intestinal infections, trichomonas, bacterial vaginosis, etc), it might help bronchitis. However, if your bronchitis is severe enough to need antibiotic treatment, metronidazole alone probably isn't enough.

Not indicated for: pneumonia and is not likely to help. Do not self treat instead see a doctor.