The most common reasons for postnasal drip (pn hereandafter) are colds, allergies, chronic sinus inflammation, esophageal reflux, especially in small kids, polyps (nasal).
Try neti pot sinus rinses. That seems to help a lot. Simple anti-allergy meds - allegra, zyrtec, claritin. If reflux is a case, may try acid reducers - zantac, (ranitidine) prilosec, etc.
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