A very bad smell from the mouth, especially a rotten egg smell even after brushing, is commonly caused by bacterial buildup in the mouth that produces sulfur-like odours. This can happen due to gum infection, plaque and tartar around the teeth, tooth decay, food particles trapped between teeth, dry mouth, poor tongue cleaning, smoking, or mouth infections. Sometimes problems such as sinus infection, tonsil stones, acid reflux, diabetes, or stomach issues can also contribute to persistent bad breath. If brushing alone is not helping, you may need professional dental cleaning to remove tartar and treat gum disease or cavities if present. Brushing twice daily, flossing regularly, cleaning the tongue gently, drinking enough water, and avoiding tobacco can help reduce the smell. Using an antibacterial mouthwash and eating fresh fruits and vegetables may also improve oral hygiene. If you have bleeding gums, loose teeth, mouth ulcers, white patches, stomach symptoms, or bad breath that continues despite good oral care, it is important to consult a dentist or doctor for proper evaluation. Treating the underlying cause usually gives the best and longest-lasting relief from bad breath.