Red eyes happen when tiny blood vessels on the surface of the eye become dilated or inflamed, making the white part look pink or bloodshot. The causes range from completely harmless (a night of poor sleep, a dusty room) to serious conditions that need immediate attention. Most cases fall somewhere in the mild-to-moderate range and clear up on their own or with simple treatment.
Allergies and Itching
If your red eyes come with intense itching, allergies are the most likely culprit. When pollen, pet dander, dust mites, or mold reach the surface of your eye, immune cells in the tissue release histamine and a cascade of inflammatory chemicals. Histamine directly dilates the blood vessels in the conjunctiva (the clear membrane covering the white of the eye), producing that classic pink, swollen look. It also disrupts the barrier lining of those blood vessels, which is why your eyelids can puff up during allergy season.
The key distinguishing feature of allergic red eye is severe itching. Viral and bacterial infections cause minimal itching by comparison. Allergic eyes also tend to produce watery or stringy mucus-like discharge rather than thick, colored pus. Both eyes are usually affected, and symptoms often track with seasonal patterns or specific environmental triggers.
Viral and Bacterial Infections
Conjunctivitis, commonly called pink eye, is one of the most frequent causes of red eyes. The type of discharge helps distinguish what’s behind it.
- Viral conjunctivitis produces a watery, clear discharge. It’s highly contagious, with a transmission risk between 10% and 50%. The virus responsible (usually an adenovirus) can survive on dry surfaces for several weeks, which is why it spreads so easily through families, classrooms, and offices. People can spread it before they even have symptoms.
- Bacterial conjunctivitis produces thicker, yellow-green, pus-like discharge that may crust your eyelids shut overnight. It’s typically self-limiting, resolving within 7 to 10 days even without treatment, though antibiotic drops can speed recovery.
Neither type causes much itching, which is what separates them from allergies. Both can start in one eye and spread to the other within a day or two.
Dry Eyes and Screen Time
Dry eye disease is one of the most underrecognized causes of chronic redness. When the tear film becomes unstable, whether from low tear production, excessive evaporation, or poor tear quality, the exposed surface of the eye triggers an inflammatory response. Immune cells release signaling molecules that dilate blood vessels and recruit more immune activity, creating a self-reinforcing cycle of dryness, inflammation, and redness.
Screen use makes this significantly worse. Under normal conditions, you blink about 14 to 16 times per minute. During focused screen work, that drops to as few as 4 to 6 blinks per minute. Some studies have recorded blink rates falling from 18 to fewer than 4 per minute during computer use. Each blink spreads a fresh layer of tears across the eye, so fewer blinks mean a drier, more irritated surface. If your eyes feel gritty, tired, and red by the end of a workday, reduced blinking is a major factor.
Broken Blood Vessels
A subconjunctival hemorrhage looks alarming: a bright red patch that covers part or all of the white of the eye. It happens when a tiny blood vessel under the conjunctiva breaks and bleeds. Despite its dramatic appearance, it’s usually painless, doesn’t affect vision, and resolves on its own within 7 to 14 days as the blood is reabsorbed.
Common triggers include hard coughing, sneezing, vomiting, straining during a bowel movement, heavy lifting, or any activity that spikes pressure in the veins of the head. In younger people, direct trauma to the eye, excessive eye rubbing, and improper contact lens use are frequent causes. No treatment is needed in most cases. The red patch may spread before it fades, shifting from bright red to yellow-green like a bruise.
Contact Lens Problems
Contact lenses sit directly on the cornea and can cause redness through several mechanisms. The lens reduces oxygen reaching the corneal surface, and over time this oxygen deprivation can trigger the growth of new blood vessels into the normally clear cornea. Lenses also reduce lubrication, create friction against the tissue, and can stimulate allergic and inflammatory responses, especially when worn too long or cleaned improperly.
If you wear contacts and notice persistent redness, the lens may be fitting poorly, trapping debris, or simply being worn for too many hours. Sleeping in lenses that aren’t designed for overnight wear is a particularly common cause of red, irritated eyes the next morning.
Eyelid Inflammation
Blepharitis, or inflammation of the eyelid margins, is a chronic condition that frequently causes red eyes even though the problem originates in the lids, not the eye itself. The eyelids and the conjunctiva meet at a junction that allows inflammation to spread easily from one to the other.
There are two main forms. In the front-of-the-lid type, bacteria colonize the base of the eyelashes and produce irritating byproducts that inflame the surrounding tissue. You may notice flaky crusts at the lash line. In the back-of-the-lid type, the tiny oil glands (meibomian glands) that line the inner eyelid become blocked or produce poor-quality oil. This increases tear evaporation, creating the same dry, inflamed surface that drives redness in dry eye disease. Chronic cases often involve both forms simultaneously, producing redness, a gritty foreign body sensation, tearing, and light sensitivity.
Uveitis and Internal Inflammation
Not all red eye comes from the surface. Uveitis is inflammation inside the eye itself, often linked to autoimmune conditions. It produces a distinctive pattern of redness concentrated in a ring around the colored part of the eye (the iris), called a ciliary flush. This looks different from the diffuse pinkness of conjunctivitis.
Uveitis typically comes with pain, blurred vision, and sensitivity to light. In more severe cases, white blood cells accumulate inside the front chamber of the eye, visible to a doctor during an exam as a foggy or hazy appearance. This type of red eye needs prompt evaluation because untreated inflammation inside the eye can damage vision permanently.
Acute Angle-Closure Glaucoma
This is the most urgent cause of red eye. Acute angle-closure glaucoma occurs when the drainage system inside the eye becomes suddenly blocked, causing pressure to spike to dangerous levels, sometimes reaching three to four times the normal range. It causes severe, sudden pain in one eye, blurred vision, halos around lights, nausea, and vomiting. The pupil is often fixed in a mid-dilated position and doesn’t react normally to light.
This condition can cause permanent vision loss within hours if untreated. It’s relatively rare compared to other causes of red eye, but the combination of sudden severe eye pain with nausea and vision changes should always be treated as an emergency.
When Red Eyes Signal Something Serious
Most red eyes are caused by allergies, dryness, or mild infections and resolve without lasting consequences. But certain combinations of symptoms point to something more dangerous. The Mayo Clinic identifies several warning signs that warrant immediate medical care: sudden changes in vision, red eye paired with a severe headache or eye pain, fever, sensitivity to light, nausea or vomiting, seeing halos around lights, swelling in or around the eye, or being unable to keep the eye open. Red eyes caused by a chemical splash or a foreign object in the eye also need prompt attention.
A useful rule of thumb: red eyes with normal vision and no significant pain are rarely emergencies. Red eyes with vision loss, severe pain, or systemic symptoms like nausea are a different situation entirely.