A red eye is most often caused by swollen or dilated blood vessels on the surface of the eye, and the vast majority of cases trace back to something minor: an infection, an allergy, or simple irritation. But in a small number of cases, redness signals something more serious, like a sudden spike in eye pressure or inflammation deep inside the eye. The key to telling the difference lies in the other symptoms that come with the redness.
Conjunctivitis: The Most Common Cause
Conjunctivitis, commonly called pink eye, is the single most frequent reason for a red eye. It affects the thin, transparent membrane covering the white of the eye and lining the inside of the eyelid. There are three main types, and each one looks and feels slightly different.
Viral conjunctivitis produces a watery discharge and a gritty sensation, sometimes with mild itching. Adenovirus is the usual culprit, and it’s highly contagious. It spreads through casual contact: shared towels, touching your face after shaking hands, even being near someone who coughs. It typically clears on its own within one to two weeks without treatment.
Bacterial conjunctivitis stands out because of its thicker, yellowish-green discharge. The most reliable sign is waking up with both eyes glued shut by dried secretions. Rather than itching, you’ll notice stinging and a foreign-body sensation, as if something is stuck under your eyelid. It spreads through direct contact with contaminated fingers and often responds to antibiotic eye drops.
Allergic conjunctivitis is not contagious at all. Its hallmark is intense itching, far more pronounced than in the infectious forms. The discharge tends to be watery or stringy. Both eyes are almost always affected, and you’ll usually have other allergy symptoms like sneezing or a runny nose alongside it.
Broken Blood Vessel on the Eye’s Surface
A subconjunctival hemorrhage looks alarming: a bright red patch spreading across the white of the eye, sometimes covering it entirely. Despite its dramatic appearance, it’s usually painless and harmless. A tiny blood vessel beneath the surface membrane ruptures and blood pools underneath.
Common triggers include coughing or sneezing forcefully, straining on the toilet, heavy lifting, rubbing your eye too hard, or even bending forward. People with high blood pressure, diabetes, or blood-clotting disorders are more prone to these bleeds. Blood-thinning medications also raise the risk. The blood reabsorbs on its own, typically within a few weeks, shifting from red to yellow-brown as it fades.
Dry Eyes and Environmental Irritation
Chronic dryness is one of the more underappreciated causes of persistent redness. When your eyes don’t produce enough tears, or the tears evaporate too quickly, the surface dries out and blood vessels dilate in response. Long hours of screen time, air conditioning, windy conditions, and low humidity all make it worse. The redness tends to be diffuse rather than patchy, and it often comes with burning, stinging, or a scratchy feeling that worsens as the day goes on.
Contact Lens Problems
Contact lenses sit directly on the cornea, and wearing them too long or caring for them improperly creates several pathways to a red eye. Lenses worn past their recommended schedule can starve the cornea of oxygen, prompting new blood vessels to grow into the normally clear tissue. This process, called neovascularization, causes visible redness that doesn’t go away when you take the lenses out.
Other contact lens complications include corneal scratches from damaged or poorly fitting lenses, inflammation of the cornea from trapped debris, and a condition specifically known as contact lens-induced acute red eye, which shows up as sudden redness and irritation, often after sleeping in lenses not designed for overnight wear. More seriously, bacteria, fungi, and even parasites can invade the eye when lenses aren’t cleaned properly, leading to infections that threaten vision.
Uveitis: Inflammation Inside the Eye
Uveitis involves inflammation of the middle layer of the eye wall rather than the outer surface. It feels fundamentally different from conjunctivitis. The pain is deeper and more aching, light sensitivity can be severe, and vision often becomes blurry. You may also notice dark floating spots drifting across your field of view. Symptoms sometimes develop gradually, but they can also come on suddenly and escalate fast.
Uveitis can stem from autoimmune diseases, infections, or eye injuries, though in many cases no clear cause is identified. It can affect one or both eyes. Left untreated, it risks permanent vision damage, so it warrants prompt evaluation by an eye specialist.
Acute Angle-Closure Glaucoma
This is the red eye emergency most people don’t know about. Acute angle-closure glaucoma occurs when the drainage system inside the eye suddenly blocks, causing pressure to spike rapidly. The symptoms are hard to miss: severe eye pain, a red eye, blurred vision, seeing rainbow-colored halos around lights, and often nausea or vomiting. The headache can be intense enough that some people initially think they’re having a migraine or even a stroke.
Unlike conjunctivitis, this condition requires emergency treatment. The elevated pressure can damage the optic nerve within hours, causing irreversible vision loss. If you develop sudden severe eye pain with nausea and vision changes, treat it as an emergency.
Other Causes Worth Knowing
Several less common conditions also produce a red eye. A corneal abrasion, or scratch on the eye’s surface, causes sharp pain, tearing, and light sensitivity. Foreign bodies lodged under the eyelid create similar symptoms along with a persistent sensation that something is stuck. Episcleritis, inflammation of the tissue between the surface membrane and the white of the eye, causes a localized patch of redness that’s mildly tender but rarely serious.
Blepharitis, a chronic inflammation of the eyelid margins, causes redness concentrated along the lash line along with crusting and irritation. Chemical splashes and trauma from impact can cause redness ranging from mild irritation to a sight-threatening emergency, depending on the substance or force involved.
When Redness Is a Warning Sign
Most red eyes resolve on their own or with minimal treatment. But certain accompanying symptoms indicate something that needs immediate attention. Sudden vision changes, severe pain, sensitivity to light, seeing halos around lights, nausea or vomiting alongside eye symptoms, and swelling in or around the eye all point toward conditions that can damage vision permanently if treatment is delayed. Redness caused by a chemical splash or an object striking the eye also warrants urgent evaluation regardless of how the eye looks afterward.
Redness-Relieving Eye Drops: A Caution
Over-the-counter drops that “get the red out” work by constricting the blood vessels on the eye’s surface. The relief is temporary. With repeated use, older formulations cause rebound redness: when the drop wears off, the blood vessels dilate even wider than before, prompting you to use the drops again in a cycle that makes the problem progressively worse. Newer formulations containing brimonidine carry a lower risk of this rebound effect, but they still don’t address the underlying cause. If you find yourself reaching for redness-relieving drops regularly, identifying and treating the root cause will serve you far better than masking the symptom.