Bacteria

• The bacteria that grows in your mouth is also a major cause of bad breath. The bacteria develops from leftover food, plaque and environmental elements that reach the inside of the mouth.
  
  When you eat, traces of food get lodged between your teeth and around your gums. When you allow that food to sit in the mouth for long periods of time, such as overnight, the odor-causing bacteria accumulates on the food. It releases a sulfur compound that causes bad-smelling breath. When someone has an untreated case of gum disease, which is an inflammation of the gums due to bad bacteria and poor dental care, bad breath is common as well.
  
  You can also have a bad breath problem when you eat foods containing a lot of garlic because garlic contains a compound called allicin. When you chop or chew garlic, the allicin is activated and released into the mouth and throat. The allicin produces an organosulfur compound, which causes the bad smell in your mouth.
  
  

Smoking

• Cigarette smoking is also a major cause of bad breath. The bad smelling tar and nicotine from cigarettes and other tobacco products builds up on the teeth and tongue. Smoking can also cause odor-causing bacteria to form. Also, because the smoke dries out the mouth, saliva doesn't flow into the mouth as readily, which facilitates the growth of bad bacteria.
  
  

Acid Reflux

• Severe acid reflux, also called GERD, can also be a cause of bad breath. Acid reflux commonly happens after eating a large meal. While it does not usually create a permanent bad breath problem, acid reflux causes foul-smelling acid to rise to your throat. When you burp, the smell emanates from your mouth and throat.
  
  

Medical Conditions

• There are some cases when brushing, flossing, or visiting the dentist don't help a bad breath problem. If there is no other source of bad breath, such as dental hygiene, gum problems, or acid reflux, the problem could be a medical condition. Blood disorders, sinus infections, diabetes or tonsillitis could cause bad breath problems. Also, lung, kidney, liver or gallbladder problems could be the issue.