What Causes Eyes to Be Red? 8 Common Reasons

Red eyes happen when tiny blood vessels on the surface of the eye dilate or become inflamed, making the normally white sclera look pink, red, or even deeply bloodshot. The causes range from something as harmless as a night of poor sleep to conditions that need prompt medical attention. In an emergency department survey in northern Iran, conjunctivitis alone accounted for about 30% of all red-eye presentations, with foreign bodies and trauma making up most of the rest.¹ Understanding which type of redness you are dealing with is the first step toward knowing whether you can wait it out or need to see someone.

1. Allergies

Allergic conjunctivitis is one of the most frequent reasons eyes turn red, especially during pollen season or in dusty environments. When airborne allergens land on the surface of the eye, they trigger immune cells called mast cells to release histamine and other inflammatory chemicals. Histamine causes the conjunctival blood vessels to widen, producing the redness, itching, and watery discharge that allergy sufferers know well.² The hallmark of allergic red eye is itching. If your eyes are red and intensely itchy but you have no pain and your vision is normal, allergies are a strong bet.

Seasonal allergic conjunctivitis flares up with tree, grass, or ragweed pollen and tends to be worse outdoors. Perennial allergic conjunctivitis, triggered by dust mites, pet dander, or mold, sticks around year-round and is often worse indoors. Both types affect both eyes equally, which helps distinguish them from infections that sometimes start in just one eye. Over-the-counter antihistamine eye drops or oral antihistamines usually bring relief, but the most effective long-term strategy is reducing contact with whatever triggers the reaction.

2. Dry Eye Disease

Dry eye is a chronic condition where the tear film either evaporates too quickly or is not produced in sufficient volume, leaving the eye surface exposed and irritated. The core problem involves tear hyperosmolarity, meaning the tears become too concentrated, which sets off a cycle of inflammation on the surface of the eye and in the tear-producing glands.³ That inflammation makes the blood vessels in the conjunctiva dilate, giving the eyes a persistent pinkish or red appearance.

What makes dry eye tricky is its self-reinforcing nature. Inflammation damages the cells that produce or stabilize tears, which makes the tears more unstable, which causes more inflammation.⁴ People with dry eye often describe a gritty or sandy sensation, burning, and eyes that paradoxically water excessively as the eye tries to compensate. The redness from dry eye tends to be mild and diffuse rather than the bright, angry red of an infection or hemorrhage. Artificial tears help in mild cases, but moderate to severe dry eye often needs prescription anti-inflammatory drops or treatments aimed at improving tear quality.

3. Infections

Eye infections are the single most common reason people show up with a red eye. They are broadly split into viral, bacterial, and, less commonly, fungal types. Viral conjunctivitis, often caused by the same adenoviruses responsible for the common cold, produces watery discharge, light sensitivity, and redness that may start in one eye and spread to the other within a day or two. Bacterial conjunctivitis tends to produce thicker, yellowish or greenish discharge and can glue the eyelids shut overnight.

In the emergency department data from northern Iran, conjunctivitis of all types was responsible for roughly 31% of red-eye cases, with viral causes alone making up nearly 20% and bacterial causes around 4%.¹ Most viral conjunctivitis resolves on its own in one to three weeks without treatment. Bacterial conjunctivitis responds to antibiotic eye drops, and starting them early shortens the illness and reduces the chance of spreading the infection. The key distinguishing feature is the type of discharge: watery points toward viral, and thick and colored points toward bacterial.

4. Broken Blood Vessel (Subconjunctival Hemorrhage)

A subconjunctival hemorrhage looks alarming. A bright red, well-defined patch appears on the white of the eye, sometimes covering a large area. It happens when a small blood vessel under the conjunctiva bursts and leaks blood into the surrounding tissue. Despite looking dramatic, it is almost always harmless and painless. In younger people, the main risk factors are eye trauma and contact lens use. In older adults, systemic conditions like high blood pressure, diabetes, and arterial disease become more relevant triggers.⁵

The blood gradually absorbs over one to three weeks, often changing from bright red to yellowish before fading completely. No treatment is needed unless the hemorrhages keep recurring, in which case it is worth checking your blood pressure and having a blood panel done to rule out clotting disorders. A single episode, especially after heavy coughing, sneezing, straining, or rubbing your eyes, is rarely cause for concern.

5. Blepharitis and Eyelid Inflammation

Blepharitis is chronic inflammation of the eyelid margins, often linked to dysfunction of the tiny oil glands (meibomian glands) that line the eyelids. These glands secrete the oily outer layer of the tear film, and when they become clogged or inflamed, tear quality suffers and the eye surface becomes irritated and red. The condition is extremely common. Estimates suggest that more than half of all patients visiting an eye clinic have signs or symptoms of blepharitis or meibomian gland dysfunction, though many are never formally diagnosed.⁶

Symptoms include crusty or flaky debris along the lash line, a burning sensation, and eyes that look mildly red most of the time. The redness tends to be concentrated near the eyelid margins and lower conjunctiva. Because it is chronic and waxes and wanes, people often get used to it and stop thinking of it as abnormal. Warm compresses applied to the closed eyelids for several minutes, followed by gentle lid massage, help unclog the oil glands. Keeping the lids clean with diluted baby shampoo or commercial lid-scrub pads is the main long-term management.

6. Environmental and Digital Irritants

Red eyes do not always trace back to a disease process. Environmental exposures are frequent culprits. Chlorinated swimming pool water, for example, damages the outermost cells of the cornea. Researchers who tested chlorinated pool-strength solutions on corneal tissue found significant increases in epithelial cell damage, visible both through dye-uptake tests and under confocal microscopy.⁷ Smoke, wind, dust, and strong chemical fumes also irritate the conjunctiva and cause transient redness.

Prolonged screen time is another widespread contributor. A survey of computer users found that about 6% reported redness as a standalone symptom, while 20% had multiple overlapping eye complaints including redness, dryness, and eye pain.⁸ The mechanism is straightforward: when you stare at a screen, your blink rate drops, sometimes by more than half. Fewer blinks means the tear film breaks down faster, exposing the surface of the eye and producing a low-grade version of the same inflammation that drives dry eye disease. Taking regular breaks, consciously blinking, and keeping the screen slightly below eye level all help reduce this type of redness.

7. Corneal Abrasion and Trauma

A scratch on the cornea, called a corneal abrasion, produces intense redness, pain, tearing, and light sensitivity. It is one of the more painful causes of a red eye because the cornea is densely packed with nerve endings. Common causes include a fingernail scraping the eye, a foreign object getting trapped under the lid, and overly aggressive contact lens handling. In the epidemiological data from emergency settings, foreign bodies and blunt or penetrating trauma combined to account for roughly a third of red-eye cases.¹

Most superficial abrasions heal on their own within 24 to 72 hours because corneal epithelial cells regenerate quickly. But the nerve damage from even a minor scratch can trigger downstream effects: research on corneal injury in animal models found that markers of nerve injury and pain signaling spike within 24 hours of a scratch, and that signs of dry eye can emerge about a week later, even after the surface has visually healed.⁹ This is why some people notice persistent dryness or mild discomfort for weeks after a seemingly minor eye injury. If you suspect an abrasion, avoid rubbing the eye and see a clinician, especially if your vision is blurred or the pain is severe.

8. Medication Side Effects

Certain medications can make your eyes red as a direct side effect, and this one catches many people off guard. Drug-induced redness of the conjunctiva is recognized across a range of topical and systemic medications. Topical eye drops are frequent offenders, and often the culprit is not the active ingredient itself but the preservatives in the solution, particularly benzalkonium chloride, which irritates the conjunctiva and cornea with repeated use.¹⁰ Glaucoma drops, some antibiotic drops, and even certain artificial tear formulations contain these preservatives.

Beyond topical drops, systemic medications including some blood pressure drugs, antihistamines (which can worsen dry eye), and certain chemotherapy agents can cause chronic redness through different mechanisms. If your eyes turned red around the time you started or changed a medication, mention it to your prescriber rather than assuming it is a coincidence. Switching to a preservative-free formulation of eye drops, when one exists, often resolves the problem without needing to stop the medication entirely.

How to Tell Which Type of Redness You Have

Not all eye redness is the same, and clinicians use a simple distinction that is worth knowing. Conjunctival injection is redness concentrated in the outer, more visible blood vessels of the conjunctiva, typically worst toward the edges and fading as you look closer to the iris. This pattern is typical of allergies, mild infections, and irritation. Ciliary injection, by contrast, is a deeper, violet-tinged redness that forms a ring around the iris and indicates inflammation of the cornea, iris, or ciliary body, conditions collectively more serious.¹¹ If you notice redness that is concentrated in a ring around the colored part of your eye rather than across the white, that is a signal to get evaluated sooner rather than later.

A few other features help narrow things down. Pain, especially deep or aching pain, suggests something beyond simple conjunctivitis. Blurred vision that does not clear with blinking is a warning sign. And pupil changes, particularly a pupil that appears smaller or does not respond normally to light, point toward inflammation inside the eye. A clinical exam that checks visual acuity, pain quality, redness pattern, pupil shape and reactivity, and eye pressure, ideally with a slit lamp, allows a much more definitive diagnosis.¹²

The Problem with Redness-Relieving Eye Drops

Over-the-counter “get-the-red-out” eye drops are enormously popular, and they do work in the short term. They contain vasoconstrictors that squeeze the dilated blood vessels shut, whitening the eye within minutes. The catch is that many of these drops work through a receptor pathway that leads to tolerance: you need more drops more often to get the same effect, and when you stop using them, the blood vessels rebound to an even wider state than before, leaving your eyes redder than when you started.¹³

This rebound redness cycle is called tachyphylaxis, and it is a genuine trap. People begin using the drops cosmetically, find that their eyes look worse whenever they skip a dose, and end up using the drops many times a day. Newer formulations based on a different receptor pathway have shown less risk of rebound in clinical trials, but the safest approach is to use redness-relieving drops sparingly and only for short periods.¹³ If your eyes are chronically red, treating the underlying cause, whether that is dry eye, allergies, or blepharitis, will always be more effective than masking the redness with a vasoconstrictor.

When Red Eyes Point to Something Systemic

Occasionally, persistent or recurring eye redness is the first visible clue to a systemic illness. Autoimmune conditions including rheumatoid arthritis, lupus, and inflammatory bowel disease can produce uveitis (inflammation inside the eye), keratitis (corneal inflammation), and other forms of ocular inflammation that present initially as a red, painful eye.¹⁴ Uveitis in particular can damage vision if left untreated, and it sometimes appears before a patient has been diagnosed with the underlying autoimmune disease.

Recurrent subconjunctival hemorrhages in older adults can also be a flag for poorly controlled hypertension or diabetes. And chronic, unexplained redness that does not respond to standard treatments sometimes prompts testing for conditions like Sjögren syndrome, which attacks moisture-producing glands throughout the body including the tear glands. The point is not that every red eye should send you running for bloodwork, but that red eyes that keep coming back despite treatment, or that are accompanied by joint pain, mouth dryness, or unexplained fatigue, are worth investigating beyond the surface.

Your Eye’s Built-In Defense System

The surface of the eye is not sterile. It hosts a community of commensal bacteria that play an active role in keeping the eye healthy. These resident microbes interact with immune receptors on the eye surface to regulate inflammatory responses, promote immune balance, and help fend off pathogenic invaders.¹⁵ When this microbial community gets disrupted, whether by aggressive antibiotic use, preservative-heavy eye drops, or chronic inflammation, the immune system can lose its footing, potentially making the eye more susceptible to infection and inflammation.

This is one reason why eye care professionals sometimes caution against using antibiotic drops “just in case” when an infection is likely viral. Unnecessary antibiotics do not help a viral infection and may disturb the protective microbial balance on the eye surface, setting the stage for future problems. It is also a reason to pay attention to the preservative content of any drops you use long term. Preservative-free formulations are gentler on both the eye surface and the microbiome that lives there.