Yeast infections are often caused by an imbalance between bacteria and the candida fungus that naturally occur on the skin. Metronidazole and other antibiotics can reduce the levels of bacteria, which can allow the fungus to grow more profusely, resulting in a yeast infection (thrush). As Metronidazole Gel is applied directly to the vagina, thrush is a common side effect. Common thrush symptoms include itching, redness and a white cottage cheese-like discharge. If you experience any of those symptoms, you should speak to a pharmacist or doctor for thrush treatment.