Allergic Rhinitis: This is the most common cause of a runny nose, often affecting one nostril more than the other due to the nasal cycle. Allergies to pollen, dust, or other environmental factors can lead to unilateral nasal discharge.

Viral Upper Respiratory Infection (URI): Common colds can cause a runny nose, which may be more pronounced on one side due to the anatomy of the nasal passages and the body's natural response to infection.
Anatomical Issues (e.g., Deviated Septum): A deviated septum can lead to uneven airflow and drainage, causing one nostril to be more congested or runny than the other.
Environmental Irritants: Exposure to smoke, chemicals, or strong odors can irritate the nasal mucosa, leading to a runny nose that may be unilateral.

Nasal Foreign Body: Especially in children, a foreign object lodged in the nostril can cause persistent, unilateral nasal discharge. This condition requires immediate medical attention to avoid complications.
Nasal Tumor or Polyp: Although rare, a tumor or polyp in the nasal cavity can cause unilateral nasal obstruction and discharge. Early detection is crucial for effective treatment.

CSF Leak: A cerebrospinal fluid leak can manifest as a clear, watery discharge from one nostril, often occurring after a head injury. This condition is rare but serious and requires prompt medical evaluation.
Fungal Infections: In immunocompromised individuals, fungal infections like aspergillosis can cause unilateral nasal symptoms, including a runny nose.
Wegener's Granulomatosis (Granulomatosis with Polyangiitis): A rare autoimmune disorder that can affect the nasal passages, leading to chronic inflammation and potentially a runny nose, among other symptoms.