Common causes of bloodshot eyes are allergic and bacterial conjunctivitis, herpes infection of he cornea, ulcers of the cornea, and dry eyes. Other things that can cause bloodshot eyes are an inflammation of the eyelid, ectropion and entropion, iritis, allergies, sty and subconjunctival hemorrhage.
Conjunctivitis, also known as pink eye, can be viral or bacterial. The conjunctiva, which is the membrane that protects the white of the eye and the inside of the eyelid, becomes inflamed. Allergies can also cause conjunctivitis.
Disorders of the eyelid can cause the eye to look bloodshot. Ectropion is a condition where the eyelid turns inside out. Usually, ectropion only affects part of the lower lid, but it leaves the inside of the eyelid vulnerable to irritants and infection. Entropion is the opposite of ectropion; the eyelid turns inward.
Subconjunctival hemorrhage presents as a bloody spot on the white of the eye. This happens when a blood vessel bursts beneath the conjunctiva. It often causes no pain, and the person frequently only notices it when he looks in a mirror. Rubbing the eyes, high blood pressure, anticoagulants, eye surgery, violent sneezing or viral infection can cause this condition.
Iritis is an inflammation of the pigmented part of the eye. Injury or disease can cause it.