This could be due to a sinus infection, a cold or a chronic
allergy. It could be sinusitis, if it is accompanied by
nasal stuffiness and facial pain or pressure. Post nasal drip could also be due to allergens. These are substances like pollen, dust, animal dander, certain foods, insect venoms and certain medications. Exposure to allergens causes a runny nose with a resultant post nasal drip where secretions from the nose go backward into the throat and irritate..