Post-nasal drip (PND), also known as upper airway cough syndrome (UACS), occurs when excessive mucus is produced by the nasal mucosa. The excess mucus accumulates in the back of the nose, and eventually in the throat once it drips down the back of the throat. It can be caused by rhinitis, sinusitis, gastroesophageal reflux disease (GERD), or by a disorder of swallowing (such as an esophageal motility disorder). Other causes can be allergy, cold, flu, and side effects from medications. However, some researchers argue that the flow of mucus down the back of the throat from the nasal cavity is a normal physiologic process that occurs in all healthy individuals. Some researchers challenge post-nasal drip as a syndrome and instead view it as a symptom, also taking into account variation across different societies. Furthermore, this rebuttal is reinforced because of the lack of an accepted definition, pathologic tissue changes, and available biochemical tests. Signs and symptoms PND may present itself through the constant presence of discomfort in the upper airways. It is classically described as the sensation of a substance "dripping down the throat" and may also present with rhinorrhea, constant throat clearing, and cough, although its symptoms can be very nonspecific. PND is one of the most common etiologies for chronic cough, defined as a cough persisting beyond 8 weeks. GERD is often associated with a high prevalence of upper-respiratory symptoms similar to those of PND, such as coughing, throat clearing, hoarseness and change in voice. Reflux causes throat irritation, leading to a sensation of increased mucus in the throat, which is believed to aggravate and, in some cases, cause post-nasal drip. Post-nasal drip can be a cause of laryngeal inflammation and hyperresponsiveness, leading to symptoms of vocal cord dysfunction. Causes There are multiple causes of PND, which can be acute or chronic. Allergic rhinitis Allergic rhinitis (AR) is a common condition where exposure to allergens results in the release of inflammatory mediators, such as histamine, that causes sneezing, rhinorrhea, itchy eyes, and nasal congestion. The increased rhinorrhea and mucus production can result in PND. Non-allergic rhinitis Non-allergic rhinitis (NAR) is a condition in which there are symptoms of rhinitis, including rhinorrhea and nasal obstruction, but with negative skin and serum allergy testing results. It can be further categorized into: Non-allergic rhinitis with eosinophilia (NARES) Hormonal rhinitis (such as during pregnancy) Medication-induced rhinitis Atrophic rhinitis Irritant and occupational rhinitis