Bad breath should first go to the dentist. According to statistics, 80%-90% of bad breath originates from the oral cavity, of which dental caries and periodontal disease is the most common cause. Dental caries caused by tooth decay, deterioration and food impaction, periodontitis caused by bleeding gums, bad restorations and residual roots and crowns of food debris, if not treated in a timely manner, after bacterial fermentation and decomposition, it will produce a bad smell. After corresponding treatment such as caries filling treatment, gingival scaling, subgingival scraping, removal of bad restorations and extraction of non-reserved replacement teeth, bad breath is still not relieved, we should continue to exclude other systemic diseases, such as pharyngitis, sinusitis, bronchitis, tracheitis, gastritis, gastrointestinal dysfunction and so on.