Brief Answer:
post nasal drip is the most common cause

Detailed Answer:
HI, thanks for using healthcare magic

Chronic cough can have different causes, according to medical studies, the 4 most common causes are: (1) post nasal cough
(2) GERD- gastroesophageal reflux disease
(3)asthma and
(4)non asthmatic eosinphilic bronchitis- associated with the presence of eosinophils (a type of white blood cell) in the lungs

Out of these, post nasal drip is the most common with 20% of persons not being aware of the drip itself. This has been found to be the cause of chronic cough in up to 80% of persons in some places.

Some persons may have more than one cause acting together to cause the cough

There are also other causes including medications.

Since post nasal drip is the most common cause, treatment is normally started here with a combination of antihistamine and decongestant. This can take 2 weeks to months to take effect.
The older drowsy antihistamines work better than the non drowsy.
A topical nasal spray plus avoiding potential causes, also helps

Topical sprays include- nasonex, nasocort, flonase, rhinocort
In terms of potential causes- commonly related to allergies eg to dust, pollen, foods. Allergy tests would determine if there is a hidden cause

If there is no improvement then treatment can be directed to one of the other causes.

I hope this helps, feel free to ask any other questions

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