Some dogs produce too much of a pigment-containing molecule that can be found in dog tears and cause the staining. When the dog’s body breaks down red blood cells, a naturally occurring molecule remains as a waste product. These molecules, known as porphyrins, contain iron, which is responsible for the red coloration, which gets darker when exposed to sunlight. Porphyrins are usually excreted via the digestive system but in dogs, they are also removed from the body in urine, saliva and tears. Darker coated dogs can also suffer from tear stains, but the marks are harder to see on the dark hair. It is a good idea to check your dog’s eyes regularly for any problems.