In cats, a red eye can be caused by inflammation of the conjunctiva or cornea, but injuries, foreign objects and other eye diseases can give similar signs. Feline herpesvirus is an important cause of recurring eye and upper airway symptoms, but not every red eye should automatically be assumed to be herpes.
Look for squinting, rubbing the eye, cloudiness, a lot of discharge or apparent sensitivity to light. The vet may need to stain the cornea to look for ulcers and assess other parts of the eye before starting treatment. This is particularly important because some eye drops can be unsuitable when the cornea is damaged.
Contact a vet if your cat seems to be in pain, is squinting or if the redness does not quickly go away. Seek help urgently for a cloudy cornea, trauma, sudden changes in sight or severe pain.