Post-nasal drip happens when mucus builds up in the back of the nose and throat. Post-nasal drip is not a medical condition, but it may be a symptom of another medical condition that causes excessive mucus production (e.g., sinusitis, rhinitis, or infections).
The inside lining of the nose produces large amounts of mucus to clean the nose, trap foreign particles (e.g., dust), and fight infection. Post-nasal drip can be caused by various medical conditions including sinusitis (inflammation of the sinuses), viral infections such as the common cold, rhinitis (a runny nose that may be acute or chronic), allergies, or bacterial infections. In some cases, post-nasal drip can be caused by reflux, or gastroesophageal reflux disease (GERD).