Bad breath, also known as halitosis, is an unpleasant odor emanating from the mouth when breathing. Under normal conditions, there is sometimes an unclean odor in the mouth. This odor is caused by a person not eating and drinking after a night’s sleep, resulting in an odor in the mouth after waking up in the morning; or by eating vegetables or other foods that contain sulfur substances. Such odor lasts for a short period of time and can be quickly eliminated by proper oral hygiene measures. In contrast, pathological bad breath is caused by disease, pathological states or tissue abnormalities in the oral cavity, and can be divided into oral and non-oral sources. Non-oral sources of bad breath are caused by some systemic diseases, such as: respiratory diseases, nasopharyngeal diseases, digestive system diseases, diabetes, uremia, leukemia, etc. Only after the eradication of systemic diseases can the problem of bad breath be solved. Most of the bad breath is of oral origin, accounting for 80%-90% of bad breath, and untreated tooth decay, gingivitis, periodontal disease and oral mucosal disease in the mouth can cause bad breath. In some cases, if the patient suffers from diseases that reduce saliva production, such as dry syndrome, tumors after radiation therapy, etc. may also form bad breath. There is also a small percentage of people who do not have bad breath at all but insist that they have bad breath, called halitophobia. Many people do not pay attention to oral hygiene and cannot brush their teeth properly on time, resulting in a large accumulation of plaque calculus and a thick tongue, which can easily cause bad breath. In the case of periodontal inflammation, there is not only a large accumulation of plaque and a significant increase in the proportion of pathogenic bacteria, but also leads to a significant increase in the amount of bacteria in the back of the tongue and saliva, and these are important causes of bad breath formation. In addition, people with a healthy mouth can also develop bad breath, and the most important source is the increase and thickening of plaque on the back of the tongue, or what we commonly call tongue moss. The huge surface area of the back of the tongue can not only store food residues, but also gather a large number of bacteria, and when the density and number of plaque on the back of the tongue is sufficient, the anaerobic bacteria can use the rich food on the tongue, hydrolyze proteins into amino acids, and then further decompose to form volatile sulfides, producing the smell of bad breath. If bad breath is caused by oral factors, such as periodontal disease, overly thick tongue, tooth decay, pericoronitis, residual roots, residual crowns, oral mucosal disease, etc., it is recommended to go to dentistry for early treatment. At present, the Department of Oral Prophylaxis of the Ninth People’s Hospital affiliated to Shanghai Jiaotong University School of Medicine has opened a specialist outpatient clinic for bad breath, which is held every Monday morning. Professional doctors and bad breath detection instruments can diagnose and treat most patients with bad breath. For bad breath caused by non-oral factors such as diseases of the respiratory system, digestive system, urinary system, and endocrine system, patients are advised to visit the relevant departments. A very small number of patients with halitosis phobia may also need the help of a psychologist. To prevent bad breath in daily life, you can do the following measures: brush your teeth properly every morning and evening and after eating, and carefully clean the gaps with dental floss; brush your teeth and also pay attention to brush your tongue in the right way; if you have dentures, you should clean them properly every day and take them off before going to bed; drink more water every day to keep your mouth moist; also avoid strong-tasting foods and reduce the high protein diet and sweet food intake. If you find that you have bad breath, you should seek medical attention, find the cause and actively cooperate with the treatment.