Most often, halitosis (bad breath) is caused by bacteria in the mouth. These bacteria create a foul odor in response to the presence of food particles or other debris in the mouth. Poor dental hygiene and a lack of saliva exacerbate the problem.

Common causes of halitosis include:

• Oral infections. The significant bacterial buildup that occurs with oral infections such as tooth decay, tooth abscess and gum disease cause halitosis.
  
  

  

• Dry mouth (xerostomia). A lack of saliva causes an abnormal dryness in the mouth and may allow the buildup of food particles and dead cells in the mouth. This can lead to halitosis. Dry mouth can occur as a side effect of certain medications, medical conditions, radiation therapy for cancer treatment or breathing through the mouth instead of the nose. Alcohol and alcohol-based mouth rinses  can also contribute to drying out the mouth.
  
  
• External agents. These may include certain foods or beverages (e.g., garlic, onions, cabbage, coffee, alcohol) and certain activities (e.g., chewing or smoking tobacco). Tobacco use can also cause dry mouth and gum disease, increasing the likelihood of halitosis among tobacco users. Oral appliances (e.g., dentures, retainer, mouth guard) can also harbor odor-causing bacteria and food particles, causing halitosis.
  
  
• Certain diets. Consuming few or no carbohydrates (sugars and starches used as energy by the body) can cause ketoacidosis (in which the body burns fat instead of carbohydrates for energy and releases odor-causing waste products in the breath), producing breath with a “fruity” or sweet smell. This can also occur during periods of fasting.

Non-oral causes of halitosis are less common. These can include:

• Respiratory tract infections. These may include throat, sinus or lung infections. Increased mucus levels that result from these infections can increase the number of odor-causing bacteria in the body and produce halitosis. For example, colds, sinus infections and many other upper respiratory infections cause post-nasal drip. Post-nasal drip allows bacteria to build up on the back of the tongue, resulting in bad breath.
  
  
• Digestive problems. A variety of conditions involving the digestive system can cause halitosis, including gastroesophageal reflux disorder (GERD) and bowel obstruction.
  
  
• Diabetes. Condition in which the body is unable to properly process sugar (glucose) in the blood. Uncontrolled diabetes can result in “fruity” smelling breath.
  
  
• Kidney disease. Conditions affecting the kidneys (which filter waste from the blood) can cause halitosis. When the kidneys fail to work properly, an ammonia-like odor can be produced in the breath.
  
  
• Nasal obstruction. When an airway passage in the nose becomes blocked, nasal discharge at the back of the throat can lead to halitosis. This occurs more often among children, due to foreign objects being lodged in the nose.