Bloodshot eyes describe a condition in which the whites of the eyes appear red. Bloodshot eyes occur when the blood vessels in the sclera (the white of the eye) dilate or swell. A variety of mild to serious diseases, disorders and conditions can irritate the blood vessels of the sclera and cause bloodshot eyes.Bloodshot eyes are often due to mild conditions, such as eye fatigue, dehydration, and overuse of contact lenses.They can also be due to infections and allergic reactions.In some cases, a bloodshot or red eye or eyes can indicate an emergent condition that can lead to loss of sight, such as acute glaucoma or an eye injury.loodshot eyes can begin suddenly and disappear quickly, such as when you have a mild allergic reaction to animal dander or dust. Bloodshot eyes can also develop with time and occur along with additional symptoms, which may be a sign of a more serious condition, such as a corneal ulcer or eye infection.