Dry Eye / KCS

Dry eye, medically called keratoconjunctivitis sicca or KCS, happens when the glands that make the watery part of your dog's tears stop producing enough of it. Every blink is meant to spread a thin film of tears across the eye to feed the cornea (which has no blood supply of its own), wash away debris, and keep everything comfortable and clear. When that watery layer runs low, the eye dries out, and the surface becomes inflamed, irritated, and vulnerable to damage.

The most common trigger is the dog's own immune system mistakenly attacking the tear glands, a process vets still don't fully understand. It can also follow certain infections, nerve damage affecting the glands, hormonal conditions like an underactive thyroid, or, importantly, some medications, particularly sulfa-based drugs, which can be toxic to the tear glands as a side effect. Certain breeds seem more prone to it, including English Bulldogs, Lhasa Apsos, Pugs, Cocker Spaniels, Shih Tzus, and several terrier breeds, and it tends to show up more in middle-aged to older dogs. It's far less commonly diagnosed in cats.

You'll usually notice it before your vet does. Signs to watch for include:

• Thick, greenish or yellow gunk collecting in the corner of the eye
• Redness of the eye and surrounding tissue
• A dull, lackluster look to the eye surface instead of its normal glossy shine
• Squinting, rubbing, or general discomfort around the eyes
• Over time, a brownish film or visible blood vessels creeping across the cornea

Vets confirm the diagnosis with a Schirmer tear test, a small strip of absorbent paper tucked against the lower eyelid for one minute to measure exactly how much tear fluid the eye produces. They'll often pair this with a fluorescein stain test, a dye that lights up any scratches or ulcers on the cornea, since dry eyes are prone to developing these painful sores.

The reason this condition matters beyond simple discomfort is what happens if it's ignored. Chronic dryness leaves the cornea unprotected, and over months this can lead to scarring, pigmentation, and new blood vessels growing across the eye's surface, changes that can permanently cloud vision. The good news is that KCS responds well to treatment in most dogs. Daily eye drops, usually cyclosporine or tacrolimus, calm the immune attack on the tear glands and often coax them back into producing tears, while lubricating drops protect the eye in the meantime. Around four in five dogs show a real improvement in tear production once treatment starts, though dogs whose Schirmer test shows very low or absent tear flow at diagnosis tend to respond less well, which is part of why catching it early makes a real difference.

KCS is a lifelong condition rather than something that resolves on its own, so ongoing management and regular vet rechecks are the norm rather than the exception. If your dog's eyes look red, goopy, or dull more often than not, it's worth getting them checked rather than assuming it's just a bit of gunk. The guides below go deeper into diagnosis, day to day management, and what to expect from treatment over time.