Why Are My Dog's Eyes Red?

Red eyes in dogs have a wide range of causes, from minor irritation to sight-threatening emergencies. The key distinguishing factors are: how red, how fast it appeared, and whether the dog seems to be in pain.

Allergies are the most common cause of mild bilateral (both eyes) redness. Environmental allergens (pollen, dust, mold) cause itchy, watery, pink eyes — similar to human hay fever. Food allergies can also cause eye irritation. The dog may rub their face or paw at their eyes.

Conjunctivitis (pink eye) causes redness with discharge — clear/watery (allergic or viral), yellow/green (bacterial), or mucoid (dry eye). Bacterial conjunctivitis usually starts in one eye and may spread to the other.

Corneal ulcer or scratch causes intense redness in one eye, squinting, tearing, and light sensitivity. Dogs may paw at the affected eye. This commonly occurs from rough play, running through brush, or a cat scratch. It needs veterinary treatment to prevent infection and deeper ulceration.

Glaucoma is the emergency to watch for: it causes rapid-onset, intense redness with a dilated, fixed pupil, cloudiness, and severe pain. The eye may appear to bulge. Glaucoma can cause permanent blindness within hours if untreated. It typically affects one eye first.

Cherry eye (prolapsed third eyelid gland) appears as a red, fleshy mass in the inner corner of the eye. It's common in young dogs and not painful but needs surgical correction.