Bad breath and yellow teeth may be caused by unhealthy diet, lack of hygiene, suffering from oral diseases and so on. 1. Unhealthy diet: usually most patients have unhealthy diet, especially like to eat spicy and stimulating food, which may make it difficult for patients’ gastrointestinal tract to digest. In addition, some of the greasy food remains on the teeth, leading to bad breath and yellowing of teeth. 2. Lack of hygiene: Generally, if patients do not pay attention to hygiene, especially at night without brushing their teeth, it may not be easy to remove the residual substances on the surface of the teeth. After a night of fermentation, not only the teeth may be damaged, but also the smell in the mouth is very unpleasant, resulting in bad breath and yellowing of the teeth. 3. Suffer from oral diseases: If the patient suffers from oral diseases, such as periodontitis, gingivitis, etc., there will be a large number of bacteria in the mouth, dental calculus, etc., which may lead to bad breath and yellowing of teeth over time. There are many other reasons for bad breath and yellow teeth, so it is recommended that patients seek timely medical attention to clarify the cause of the disease and then carry out regular treatment.