What causes tear stains in dogs, and how do I get rid of them?

Quick answer

A compound in tears called porphyrin oxidizes on contact with air, causing reddish staining and odor. Causes range from food allergies (in food or treats), a blocked tear duct (epiphora), to fine dust or an eyelash poking the eye. Try switching to a hydrolyzed food that limits allergenic ingredients (like meat proteins), and wipe moisture around the eyes daily with dry gauze to keep the area dry.