Thanks for your question.
Basically you must realize that a bad breath can be due to a disorder of the oral cavity, nasal passages, sinuses, respiratory tract, or gastrointestinal tract.
Actually this symptom is very common and patients are frequently embarrassed and may take measures to hide it.
A mothball smell can be due to tooth decay. Bacteria act on the trapped food, and degrade it releasing gases like ammonia and enzymes that vause the mothball odor. But since you had a recent dental consultation, other causes need to be considered.
There is always a cause behind this symptom and a physical examination by a doctor will definitely be helpful. Frequently conditions such as large tonsils, sinusitis, or reflux can cause this symptom. Hence, though a good oral hygiene is helpful, but will not be beneficial if the causative factor is not due to oral conditions.
Therefore I would suggest a consultation with an ENT doctor who is the specialist to consult for this symptom (if oral causes have been ruled out) and he/she will do a thorough physical examination and based on the causative factor, treatment will resolve this symptom. If this is due to reflux then medications such as prilosec are helpful.
Since you say that this smells like ammonia, your doctor may also suggest liver or renal function tests also, if indicated.
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Edited by Dr. Ravoof on 10/2/2009 at 12:38 AM EST