According to the Mayo Clinic, a constantly runny nose may be caused by anything that inflames or irritates the nasal tissues. Examples of common inflammations or irritations include dry air, pet and plant allergies, mold and peanut allergies, exposure to bright lights, chronic and acute sinusitis and other factors.
Less common triggers for a constantly running nose are polyps or tumors in the nose, cluster or migraine headaches, Churg-Strauss syndrome and RSV (respiratory syncytial virus). Some people have a chronically runny nose for no apparent reason, a condition called non-allergic rhinitis or vasomotor rhinitis. The Mayo Clinic recommends you work with your health care professional for an accurate diagnosis of a constantly runny nose.