Cleveland Clinic on Instagram: "Several health conditions — some more serious than others — can make your nose run constantly. A chronic or long-term runny nose is known as chronic rhinorrhea. Here are 7 causes, explained: Allergies: They’re the most common reason for a constant runny nose. Allergies happen when your immune system mistakes a harmless substance, like pollen, dust or pet dander, for a dangerous intruder. Your body responds by bringing on lots of snot to trap the invaders. Nonaller