Allergic Rhinitis: This is the most common cause of post nasal drip, often associated with other symptoms like sneezing, runny nose, and itchy eyes. The continuous nature of the symptom aligns with the chronic exposure to allergens seen in allergic rhinitis.

Viral Upper Respiratory Infection (URTI): Common colds can cause post nasal drip due to the increased production of mucus. This is usually self-limiting but can be recurrent.
Non-Allergic Rhinitis: This includes a variety of conditions that cause chronic rhinitis symptoms without an identifiable allergy. It can be due to environmental factors, hormonal changes, or certain medications.
Sinusitis: Both acute and chronic sinusitis can cause post nasal drip due to the infection and inflammation of the sinuses leading to increased mucus production.
Gastroesophageal Reflux Disease (GERD): Stomach acid flowing back into the throat can cause irritation and lead to post nasal drip, especially at night.

Nasopharyngeal Cancer: Although rare, it can present with persistent post nasal drip among other symptoms like nasal obstruction and bleeding. Early detection is crucial for treatment.
Foreign Body: Especially in children, a foreign body in the nose can cause persistent post nasal drip and should be ruled out to prevent complications.
CSF Leak: A cerebrospinal fluid leak can present with clear, watery post nasal drip, which is a medical emergency requiring prompt diagnosis and treatment.

Primary Ciliary Dyskinesia: A genetic disorder affecting the cilia in the respiratory tract, leading to chronic respiratory infections and potentially post nasal drip.
Young's Syndrome: A condition characterized by chronic rhinosinusitis, nasal polyposis, and bronchiectasis, often associated with post nasal drip.
Sjögren's Syndrome: An autoimmune disorder that can cause dryness in the eyes and mouth but also affect the respiratory system, leading to post nasal drip in some cases.