Allergic Rhinitis: This is the most common cause of a constant runny nose, often triggered by allergens such as pollen, dust mites, or pet dander, leading to chronic inflammation and nasal discharge.

Viral Upper Respiratory Tract Infection (URTI): Common colds can cause a runny nose due to viral infection of the nasal mucosa, leading to increased mucus production.
Non-Allergic Rhinitis: Includes various forms of rhinitis not caused by allergies, such as vasomotor rhinitis, where the nasal blood vessels are overly sensitive, or hormonal rhinitis, related to hormonal changes.
Sinusitis: Inflammation or infection of the sinuses can lead to a constant runny nose, as the sinuses drain into the nasal cavity.

Nasal Tumor or Cancer: Although rare, a tumor in the nasal cavity can cause persistent nasal discharge. Early detection is crucial for effective treatment.
CSF Leak: A cerebrospinal fluid leak can present with a constant, clear nasal discharge, which is a serious condition requiring prompt medical attention.
Granulomatosis with Polyangiitis (GPA): A form of vasculitis that can affect the nasal passages and sinuses, leading to chronic nasal discharge and destruction of nasal tissue.

Primary Ciliary Dyskinesia: A genetic disorder affecting the cilia in the respiratory tract, leading to chronic respiratory infections and nasal discharge.
Sjögren's Syndrome: An autoimmune disorder that can cause dryness in the eyes and mouth but sometimes presents with nasal symptoms due to inflammation of the nasal mucosa.
Fungal Sinusitis: A fungal infection of the sinuses, more common in immunocompromised individuals, which can cause chronic nasal discharge and other sinus symptoms.