About 10% of women report a vaginal yeast infection (vaginal candidiasis) as a side effect of metronidazole treatment. Although metronidazole spares vaginal lactobacilli, which are naturally resistant to nitroimidazoles such as metronidazole, it does not restore a Lactobacillus-dominated microbiome and disruption in the normal vaginal microbiota can persist.

This can result in an overgrowth of the candida yeast. Symptoms of a yeast infection include itching and a thick, white odor-free discharge and it can be easily treated with medication such as fluconazole.

Oral yeast infections (oral thrush or oral candidiasis) have also been reported with metronidazole oral treatment. Symptoms may include creamy white lesions on the tongue, inner cheeks, or sometimes on the roof of your mouth, gums and tonsils; mouth redness, burning, or soreness and a furry or swollen tongue. Treatment is with an antifungal mouthwash such as nystatin or oral antifungal tablets or capsules.