Susan, metronidazole is prepared for either a topical preparation for
rosacea, or it comes as a vaginal cream for bacterial vaginosis.
Metronidazole has a fairly limited spectrum of activity as an
antibiotic: it's really good for anerobes. If I ever tried it, I would
use the vaginal preparation, as it's made to be applied to mucosa. For
sinusitis, metronidazole is usually used with another broader spectrum
antibiotic: like a cephaloporin plus flagyl.
 Recently, I've seen too much clostridium difficile colitis and we use
it for that: an overgrowth colitis caused by antibiotic use: but
recently starting to pop up even when antibiotics weren't used. Also,
it's getting more virulent. I spoke a GI specialist who had a really
interesting take on it: as c.diff is a spore forming bacteria, she
first treats with a 10 day course and then weans patients off: two days
on and one day off, to let the spores bloom.(Completely off target from
your question...)
Judy