Post-nasal drip (PND, or post nasal drip syndrome, PNDS, also known as Upper Airways Cough Syndrome, UACS) occurs when excessive mucus is produced by the nasal mucosa. The excess mucus accumulates in the throat or back of the nose. It is caused by rhinitis, sinusitis, gastroesophageal reflux disease (GERD), or by a disorder of swallowing (such as an esophageal motility disorder). It is frequently caused by an allergy, which may be seasonal or persistent throughout the year.