Although you may want to consult your physician if you have a particular problem with bad breath, the following general advice will give you insight into the likeliest cause of a bad breath problem.

The principal bad breath cause is associated with bacteria in the mouth. This can flourish when the saliva flow in the mouth is reduced. Lack of saliva means that old food is not washed away, and saliva also helps in neutralizing acids. Saliva also performs the very necessary task of flushing out the mouth, and removing food particles, so any reduction in saliva flow inhibits this action.

Saliva can be reduced for a number of reasons. There are natural causes of this reduction, and other causes related to disease or choices. The flow of saliva reduces naturally as we get older, making us more prone to bad breath. Also when we are dehydrated, the moisture in the mouth reduces naturally. Often when asleep, people will have their mouths open, maybe even snoring, and this will dry out the mouth. This contributes to the "morning breath" syndrome.

Certain diseases and illnesses can similarly contribute to bad breath. Any infection that affects the respiratory tract can be a cause of bad breath, as well as many other diseases.

Dry mouth can also be caused by medications for various problems. The most common is probably the antihistamine, taken to relieve allergies. Some others include high blood pressure and depression medicines. Other things that can be ingested that affect the dryness of the mouth include alcohol and mouthwashes that contain alcohol.

One of the many problems associated with cigarette smoking is dry mouth. For many reasons unrelated to bad breath, it is wise to avoid smoking, as has been documented extensively elsewhere. With a predisposition to bad breath, however, the habit of smoking virtually guarantees that you will suffer from bad breath.

It is no good to think that the bacteria causing bad breath may be generally eliminated, as they are naturally occurring and required for bodily function. They are anaerobic, which means "without oxygen". As saliva provides oxygen the lack of saliva with a dry mouth makes a more favorable environment for the bacteria to increase, aggravating the bad breath.

The bacteria create sulfurous compounds, such as hydrogen sulfide (the bad egg smell) which are the cause of the smelly breath. The bacteria assist in the digestive process by starting the breakdown of proteins. Thus another cause of bad breath would be a high protein diet, which feeds the bacteria.