Allergic or bacterial conjunctivitis, dry eyes, hay fever or a sty within the eye can cause red, bloodshot eyes, according to Mayo Clinic. People with an eye injury or foreign object in the eye typically have red, bloodshot eyes in addition to discomfort near the eye.
Certain eye conditions or illnesses, such as a corneal abrasion, corneal herpetic infections, inflammation of the membrane that covers the white part of the eye, or inflammation of the cornea, can cause red, bloodshot eyes, explains Mayo Clinic. An infection of the tissues around the eye and conditions where the eyelids turn out or in sometimes cause red, bloodshot eyes. Patients with viral conjunctivitis, corneal ulcers and subconjunctival hemorrhage may experience red, bloodshot eyes, too.
Pink eye, a bacterial and viral infection that affects the eye, causes eyes to become red and bloodshot, according to Mayo Clinic. Pink eye may also produce pus, mucous and clear discharge from the eye. The condition is contagious until the individual receives medical treatment.
Another cause of bloodshot eyes is acute glaucoma, which is a sudden increase in pressure. Acute glaucoma is typically chronic and gradual and is a medical emergency.
Conditions that cause red, bloodshot eyes can affect one or both eyes, explains Mayo Clinic. Redness typically develops when the blood vessels expand on the surface of the eye due to inflammation because of an infection or irritation within the eye.