About 10% of women report a vaginal yeast infection (vaginal candidiasis) as a side effect of metronidazole treatment. This is because metronidazole not only kills the bacteria responsible for conditions such as bacterial vaginosis, but useful bacteria in the vaginal flora that help to keep other microbes in check. 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.