Will metronidazole treat the bad odour coming from my sinuses?

Maybe.: Flagyl (metronidazole) kills some bacteria, & other organisms, that can cause chronic sinusitis. It's usually tried in combination with another drug. We don't usually use it at first, with acute sinusitis, though, unless we have a reason to try it before other drugs (like allergies to other meds, cause of sinusitis, if known, foreign travel, etc.) Don't drink alcohol when taking it! Ask doc why.