Brief Answer:
allergies - the most common reason for persistent post nasal drip and cough

Detailed Answer:
HI, thanks for using healthcare magic

It is definitely possible that allergies are contributing to his symptoms since allergies are the most common reason for post nasal drip.

It can be an allergy to food, drink, medication, environmental (pollen, animal dander, odours), chemicals.
If possible, an allergy test (either blood or skin), would be best to find the exact allergen.

Infection is not likely since the cough has been ongoing for years.

Treatment of post nasal drip includes: (1) reducing exposure to the cause if possible- I had one patient whose allergy tests showed that she was allergic to bananas, this was the cause of her drip

(2)use of an anti histamine/decongestant combination even if there is no nasal congestion
This combination of medication helps to reduce the post nasal secretions
Eg zyrtec, d, claritine d, allegra d, benadryl d

(3)a topical steroid nasal spray is sometimes also prescribed eg nasonex, nasocort, rhinocort, flonase

(4)medications such as singulair or ketotifen which work along a different pathway than anti histamines, are sometimes also prescribed along with the antihistamines/decongestant

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

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