Red blood vessels in the eyes are almost always caused by irritation, inflammation, or a burst vessel in the thin membrane covering the white of your eye. The most common culprits are dry eyes, allergies, infections, lack of sleep, and broken blood vessels from straining or coughing. Most causes are harmless and resolve on their own, but a few deserve prompt attention.
How Blood Vessels Become Visible
The white of your eye is covered by a thin, transparent membrane called the conjunctiva, which is packed with tiny blood vessels. Normally these vessels are so small you can’t see them. When something irritates or inflames the eye, those vessels dilate and fill with more blood, making them suddenly visible as red lines or patches. This process, called conjunctival hyperemia, is a vasodilatory response triggered by inflammation from a wide range of infectious and non-infectious causes.
A different type of redness happens when one of those tiny vessels actually bursts. Blood pools under the conjunctiva and creates a bright red spot that looks alarming but is usually painless. This is called a subconjunctival hemorrhage, and it’s the eye equivalent of a bruise.
Everyday Causes
The reasons your eyes look red on any given day tend to be mundane. Dry eye syndrome means your eyes either don’t produce enough tears or the tears evaporate too quickly, leaving the surface poorly lubricated. That dryness triggers inflammation, producing a scratchy, burning sensation along with visible redness. Staring at screens for long stretches makes it worse because you blink less, which speeds up tear evaporation.
Allergies are another extremely common trigger. When your eyes encounter pollen, pet dander, or dust mites, your immune system releases histamine, which causes the blood vessels to swell and the conjunctiva to become inflamed. The hallmark that separates allergies from dry eye is intense itching. Allergic conjunctivitis also tends to come with watery discharge, puffy eyelids, and sometimes a runny nose. Dry eyes, by contrast, feel more like grit or stinging, with milder itching if any.
Sleep deprivation, alcohol, smoke exposure, and wind can all cause temporary redness by irritating or dehydrating the eye surface.
Broken Blood Vessels
A subconjunctival hemorrhage looks dramatic: a solid patch of bright red on the white of your eye. It happens when a tiny vessel ruptures, and the blood has nowhere to go, so it spreads under the conjunctiva. Common triggers include coughing, sneezing, vomiting, straining on the toilet, rubbing your eyes, or a minor bump to the eye.
Several health conditions raise your risk. High blood pressure and diabetes both weaken small blood vessels over time, making them more prone to breaking. Blood-thinning medications like warfarin and aspirin also increase the likelihood. Bleeding disorders such as hemophilia or von Willebrand disease make these hemorrhages more frequent as well.
Despite how it looks, a subconjunctival hemorrhage is painless and doesn’t affect your vision. It fades over one to three weeks as the blood is reabsorbed, often shifting from red to yellow before disappearing completely, much like a bruise on your skin.
Infections and Pink Eye
Conjunctivitis, commonly called pink eye, is an infection or inflammation of the conjunctiva that causes diffuse redness across the white of the eye. Viral conjunctivitis is the most common form, spreading easily through hand-to-eye contact and producing watery discharge, light sensitivity, and a gritty feeling. It typically clears on its own within one to two weeks.
Bacterial conjunctivitis produces thicker, yellowish-green discharge and can cause the eyelids to stick together in the morning. It often responds to antibiotic eye drops. Both types are contagious, so frequent hand washing and avoiding shared towels matter during the active phase.
Contact Lens Complications
Wearing contact lenses introduces several risks that can turn your eyes red. Lenses sit directly on the cornea and reduce the amount of oxygen reaching it. This oxygen deprivation, called corneal hypoxia, can cause swelling of the cornea and trigger inflammation. Over time, chronic oxygen deprivation can even stimulate new blood vessels to grow into the cornea, a process called neovascularization, as the eye tries to compensate for the lack of oxygen.
Contact lenses also create friction against the conjunctiva, promote dryness, and can trigger allergic reactions to lens materials or cleaning solutions. Overwearing lenses, sleeping in lenses not designed for overnight use, or poor hygiene increases the risk of microbial keratitis, a corneal infection that causes pain, redness, and light sensitivity. People who wear contact lenses also have a higher rate of subconjunctival hemorrhages because of the added friction and dryness.
Serious Conditions That Cause Red Eyes
Most red eyes are benign, but certain patterns of redness signal something that needs immediate care.
- Acute angle-closure glaucoma is a medical emergency. Pressure inside the eye spikes rapidly, causing severe eye pain, blurred vision, halos around lights, nausea, vomiting, and a red eye. Symptoms are more common at night. Without prompt treatment, permanent vision loss can occur quickly.
- Anterior uveitis (iritis) is inflammation inside the eye itself, not just on the surface. It produces a deep, aching pain that develops over hours, light sensitivity, blurred vision, and redness concentrated around the colored part of the eye. This needs evaluation within 24 to 48 hours.
- Orbital cellulitis is a bacterial infection of the tissue around the eye. It causes redness, swelling, warmth, fever, pain with eye movement, and sometimes the eye appears pushed forward. This requires emergency hospital treatment.
- Chemical burns from household cleaners, solvents, or industrial chemicals cause immediate redness, blurry vision, and pain. Flushing the eye with clean water for at least 15 to 20 minutes before getting emergency care is critical.
Episcleritis, an inflammation of the tissue just beneath the conjunctiva, causes a localized patch of redness that can be mildly tender. It usually resolves on its own within three weeks, though recurrent or persistent episodes warrant evaluation to rule out scleritis, a more serious condition marked by intense, boring pain and a blue-purple discoloration of the white of the eye.
When Redness Is a Red Flag
The features that separate harmless redness from something urgent are pain, vision changes, and light sensitivity. A red eye that feels fine, sees fine, and isn’t getting worse is rarely dangerous. A red eye paired with severe pain, sudden blurred or decreased vision, sensitivity to light, halos around lights, nausea, or a recent injury to the eye or head warrants same-day evaluation.
Discharge matters too. Clear, watery discharge suggests a viral cause or allergies. Thick, colored discharge points toward a bacterial infection. No discharge at all, combined with deep pain, is more concerning for conditions like uveitis or glaucoma. If redness keeps recurring without a clear trigger like allergies or dry eyes, that pattern itself is worth investigating, since repeated episodes can sometimes reflect underlying vascular conditions, autoimmune disease, or chronically elevated blood pressure.