Post-nasal drip occurs when mucus drains down the back of your throat. It is a common symptom that can be caused by a variety of conditions. Seasonal allergies and colds are some of the most common causes of post-nasal drip. Post-nasal drip can be irritating and lead to coughing, throat clearing, and a sore throat due to the irritation. Many patients report that post-nasal drip worsens at night, which occurs because of the change in ...