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.
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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You are welcome.
Wishing you best of health and happiness.