Red eyes happen when tiny blood vessels on the surface of your eye widen and fill with more blood than usual. This can be triggered by anything from allergies and dry air to infections and contact lens irritation. Most causes are harmless and resolve on their own, but certain combinations of symptoms point to conditions that need prompt attention.
What Happens Inside a Red Eye
The white of your eye is covered by a thin, transparent membrane called the conjunctiva, which is packed with microscopic blood vessels. Normally these vessels are so small you can barely see them. When something irritates or inflames the eye, your body releases chemical signals that cause the smooth muscle around those vessels to relax. The vessels expand, fill with blood, and suddenly become visible as pink or red patches.
Different triggers use different chemical pathways to cause this expansion. During an allergic reaction, your body releases histamine, which acts on the vessel walls to produce nitric oxide, a molecule that relaxes the surrounding muscle. During an infection, immune cells release inflammatory proteins that do essentially the same thing through a separate route. Your nervous system can also trigger redness directly: sensory nerves in the eye detect an irritant and relay a signal to the brain, which sends back a response that releases chemicals at the eye’s surface, dilating the vessels. All three pathways lead to the same visible result.
Allergies, Infections, and Pink Eye
Conjunctivitis, commonly called pink eye, is one of the most frequent reasons for red eyes. The type of discharge you see is the most reliable way to tell which kind you’re dealing with.
- Bacterial conjunctivitis produces a thick, white-yellow discharge that tends to crust your eyelashes together overnight. It often feels like something is stuck in your eye. The discharge reforms quickly after you wipe it away.
- Viral conjunctivitis produces a watery, clear discharge with tearing. It often follows or accompanies a cold or upper respiratory infection. The lymph node in front of your ear may feel swollen and tender on the affected side.
- Allergic conjunctivitis also produces watery discharge, but the dominant symptom is itching and burning. Eyelids may look puffy. There’s typically a history of seasonal allergies or a known trigger like pet dander.
Viral and allergic conjunctivitis generally clear on their own. Bacterial cases sometimes need antibiotic eye drops, especially if the discharge is heavy or worsening after a few days.
Dry Eyes and Chronic Redness
Dry eye disease is a leading cause of persistent, low-grade redness that doesn’t seem tied to a specific event. It starts when your tear film becomes unstable, either because you don’t produce enough tears or because they evaporate too quickly. Without that protective layer, the eye’s surface is exposed to air, friction from blinking, and environmental irritants.
That exposure kicks off an inflammatory cycle. Immune cells on the eye’s surface activate, releasing inflammatory molecules that damage the surface tissue. This damage then signals more immune cells to respond, creating a self-sustaining loop of inflammation. Over time, the immune system generates specialized cells that migrate back to the eye and perpetuate the problem. This is why dry eye often feels like it gets worse over months or years rather than appearing suddenly.
Common contributors include long hours of screen use (which reduces your blink rate), air conditioning, low humidity, and aging. Wind and air pollution also strip moisture from the eye’s surface.
Environmental Irritants
Air pollution is a surprisingly potent trigger for red eyes. Ground-level ozone, a common pollutant in urban areas, acts as a free radical on the eye’s surface, generating reactive molecules that provoke inflammation and swelling of the conjunctival blood vessels. Fine particulate matter (the type measured as PM2.5 in air quality reports) has been shown to damage the tear film, increase corneal inflammation, and reduce tear volume. Diesel exhaust nanoparticles cause a direct inflammatory response in the conjunctival tissue.
Chlorine in swimming pools can chemically irritate the eye’s surface, and wildfire smoke carries a mix of fine particles and gases that compound the effect. Even nitrogen dioxide from traffic exhaust has been linked to dry eye and conjunctivitis in areas with poor air quality.
Contact Lens Problems
Contact lenses sit directly on the cornea, so any issue with fit, hygiene, or wearing schedule can quickly lead to redness. Two complications are particularly worth knowing about.
Giant papillary conjunctivitis is an allergic reaction to protein deposits that build up on the lens surface. It causes itching, redness, excess mucus, and a growing intolerance to wearing lenses. The underside of the upper eyelid develops large, cobblestone-like bumps. Treatment usually means stopping lens wear for one to two months, switching to a different lens type, and using anti-inflammatory drops.
Microbial keratitis is less common but more serious. It occurs when bacteria or other microorganisms invade the cornea through a tiny scratch or oxygen-deprived area under the lens. The most frequent culprit is Pseudomonas aeruginosa, a bacterium that thrives in contaminated lens cases and cleaning solutions. Symptoms include pain, redness, and discharge that seem disproportionately severe. Poor lens hygiene, sleeping in contacts, and wearing lenses with pre-existing dry eyes all raise the risk.
Broken Blood Vessels
A subconjunctival hemorrhage looks alarming: a bright red, well-defined patch on the white of the eye, sometimes covering a large area. Despite its appearance, it’s usually painless and harmless. A small blood vessel under the conjunctiva simply bursts and leaks blood into the surrounding space.
Common triggers include coughing, sneezing, vomiting, heavy lifting, straining, or rubbing the eye too hard. In younger people, it’s often linked to minor trauma or contact lens use. In older adults, it tends to happen spontaneously and is more common in those with high blood pressure, diabetes, or who take blood-thinning medications. Most cases resolve within 7 to 14 days as the body reabsorbs the blood, shifting in color from red to yellow-brown before fading completely.
Medications That Cause Red Eyes
Several common prescription drugs list eye redness or conjunctivitis among their side effects. Beta-blockers used for blood pressure can cause dry eye syndrome and conjunctival inflammation. ACE inhibitors have been associated with conjunctivitis and light sensitivity. Certain cancer immunotherapy drugs can trigger a range of eye inflammation, from mild conjunctivitis to more serious conditions like uveitis and scleritis. Even bone-density drugs (bisphosphonates) used for osteoporosis have been linked to conjunctivitis and deeper eye inflammation. If your eyes became persistently red after starting a new medication, it’s worth flagging for your prescriber.
When Red Eyes Signal Something Serious
Most red eyes are surface-level problems. But redness combined with certain other symptoms can indicate conditions that threaten your vision if untreated.
Uveitis is inflammation of the middle layer of the eye wall. It causes redness along with eye pain, light sensitivity, blurred vision, and dark floating spots. Symptoms can come on suddenly and worsen quickly, or develop gradually over days. It sometimes affects only one eye. Left untreated, uveitis can lead to permanent vision loss.
Acute angle-closure glaucoma causes sudden redness with severe eye pain, headache, nausea or vomiting, and seeing halos or colored rings around lights. This is a medical emergency requiring treatment within hours to prevent optic nerve damage.
Scleritis, inflammation of the tough outer wall of the eye itself, produces deep, boring pain that worsens with eye movement and can radiate to the forehead or jaw. Corneal ulcers, often a complication of contact lens infections, cause intense pain, tearing, and a visible white spot on the cornea.
The pattern to watch for: redness paired with significant pain, sudden vision changes, light sensitivity, nausea, or halos around lights. Any of these combinations warrants immediate evaluation, not a wait-and-see approach.