Most of us have experienced bloodshot or red eyes. They can look unpleasant, with pink or red streaks in the conjunctiva and sclera, which are usually white. Bloodshot eyes can also be uncomfortable—with itchiness, tearing, and burning all common symptoms. Red eyes are usually caused by an external irritant (called an allergen). To fight off the irritant, your eyes make a substance called histamine, which dilates and inflames the tiny blood vessels under the eye’s surface.

Many cases of red eye are harmless and respond well to home or over-the-counter treatments. The best thing you can do is find what allergen is causing the red eye, and avoid it. But some causes of red eye require medical diagnosis and prescription medication.