What are bloodshot eyes? 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. Bloodshot 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. Depending on the underlying cause, treatment for bloodshot eyes can range from rest and hydration to more extensive treatment, such as medication or surgery. Because bloodshot eyes may be a sign of a serious condition, contact your medical professional about your symptoms. Seek prompt medical care if you have unexplained, persistent, or severely bloodshot eyes or if you are concerned about your symptoms. Seek immediate medical care (call 911) if you have bloodshot eyes accompanied by severe eye pain, loss of vision, or signs of anaphylactic shock (swollen tongue and throat, hives, and difficulty breathing).