A red eye happens when tiny blood vessels on the surface of your eye widen and fill with more blood than usual. This is your eye’s version of inflammation, and it can be triggered by anything from allergies and dry air to a serious infection or pressure buildup inside the eye. Most causes are harmless and resolve on their own, but a few need prompt attention.
Why Blood Vessels in Your Eye Dilate
The white of your eye is covered by a thin, clear membrane called the conjunctiva, which contains hundreds of tiny blood vessels. When something irritates or inflames your eye, your immune system releases chemical signals, primarily histamine and other inflammatory molecules, that cause these vessels to relax and expand. More blood flows in, and the white of your eye turns pink or red. This process can be triggered by allergens landing on your eye, bacteria or viruses setting up an infection, or even physical rubbing and dryness.
Your nervous system can also drive redness directly. Sensory nerves in the conjunctiva detect irritation and relay signals that cause local release of compounds that widen blood vessels. This is why something as simple as a gust of wind, smoke, or staring at a screen too long can leave your eyes looking bloodshot.
Conjunctivitis (Pink Eye)
Pink eye is the most common cause of a red eye, and it comes in three forms that look and feel different from each other.
Viral conjunctivitis is the most frequent type in adults. It often accompanies a cold, sore throat, or respiratory infection. The discharge tends to be watery rather than thick, and it usually starts in one eye before spreading to the other. It’s highly contagious, spread mainly through hand-to-eye contact, and clears up on its own without antibiotics.
Bacterial conjunctivitis produces a thicker, yellow-green discharge that can glue your eyelids shut overnight. You may notice more swelling around the lids and a gritty discomfort. It’s also very contagious. Antibiotic eye drops speed recovery, though mild cases can resolve without them.
Allergic conjunctivitis is not contagious at all. It tends to affect both eyes at once and causes intense itching, tearing, and puffiness. It’s most common in people who also have hay fever, asthma, or eczema, and it follows seasonal or environmental allergen exposure.
Burst Blood Vessel
A subconjunctival hemorrhage looks alarming: a bright red patch appears on the white of your eye, sometimes covering a large area. Despite its appearance, it’s almost always painless and harmless. A small vessel simply ruptures and bleeds underneath the conjunctiva.
Common triggers include sneezing, coughing, straining during a bowel movement, heavy lifting, or rubbing your eyes too hard. Contact lens irritation and minor eye injuries can also cause it. People taking blood thinners or those with high blood pressure or diabetes are at higher risk. The blood absorbs on its own within 7 to 14 days, shifting from red to yellow-brown as it fades, similar to a bruise. No treatment is needed.
Screen Time, Dry Air, and Contact Lenses
Chronic, low-grade redness often has an environmental explanation. As little as two hours of continuous screen use can bring on symptoms of digital eye strain, and sessions longer than four hours significantly increase the risk. Staring at a screen reduces your blink rate, which means your tear film evaporates faster and leaves the surface of your eye exposed and irritated. Smartphones are worse than larger screens because the smaller display demands more intense focus at a closer distance.
Dry indoor air, air conditioning, poor lighting, and screen glare all compound the problem. Contact lens wearers are especially vulnerable. Surveys show that 89% of soft contact lens wearers report eye fatigue at least once a month, and over 60% experience dryness and irritation weekly. If your redness tends to worsen through the day or feels better on weekends away from screens, these lifestyle factors are the most likely culprits.
Uveitis: Inflammation Inside the Eye
Uveitis is inflammation of the middle layer of the eye wall, and it can mimic pink eye at first glance. The key differences are important. Uveitis causes deep, aching pain and sensitivity to light but does not produce the burning, gritty feeling, tearing, lid swelling, or discharge that conjunctivitis does. It can also cause floaters, small specks or threads drifting across your vision, which conjunctivitis never does. Vision may become noticeably blurry.
Because uveitis affects structures deeper inside the eye, it requires prompt treatment to prevent lasting damage. If your red eye comes with light sensitivity, floaters, or blurred vision but no discharge or crusting, the cause is likely not simple pink eye.
Red Flags That Need Urgent Attention
Most red eyes are minor irritations, but certain symptoms point to conditions that can threaten your vision. Pain is the single most important distinguishing feature. A red eye with only mild scratchiness or irritation is rarely dangerous. A red eye with moderate to severe pain raises the possibility of a corneal infection, uveitis, acute angle-closure glaucoma, or scleritis.
Acute angle-closure glaucoma happens when fluid pressure inside the eye spikes suddenly. It causes intense pain, headache, nausea or vomiting, halos around lights, and blurred vision. The pupil on the affected side may look larger than the other and won’t react normally to light. This is a true emergency that can cause permanent vision loss within hours if untreated.
Scleritis, inflammation of the tough outer wall of the eye, produces severe, boring pain that worsens at night and with eye movement. The pain can radiate into the face and jaw. Without treatment, it can erode through the eye wall.
In general, seek urgent care if your red eye comes with any combination of these:
- Moderate or severe pain
- Sensitivity to light
- Decreased or blurry vision
- Halos around lights
- Nausea or vomiting alongside eye pain
- A pupil that looks abnormal compared to the other eye
Over-the-Counter Redness Drops
Redness-relief eye drops work by constricting the dilated blood vessels on the surface of your eye. They’re effective in the short term, but the type of drop matters more than most people realize.
Traditional over-the-counter drops (containing ingredients like tetrahydrozoline) constrict both the small arteries and veins on your eye’s surface. The problem is that squeezing arteries reduces oxygen delivery to the tissue. After as few as 5 to 10 days of daily use, you can develop tolerance, meaning the drops stop working as well. Worse, when you stop using them, the oxygen-starved vessels can dilate even wider than before, creating “rebound redness” that looks worse than what you started with.
Newer formulations containing a selective ingredient (brimonidine 0.025%) work primarily on the veins rather than arteries. Because oxygen-carrying arteries aren’t being squeezed shut, ischemia doesn’t develop. In clinical trials, rebound redness occurred in only about 1% of users, and tolerance with long-term use was not observed. If you need a redness drop occasionally, this distinction is worth knowing when choosing a product.
For redness caused by allergies, antihistamine eye drops address the underlying trigger rather than just masking the symptom. For dryness-related redness, preservative-free artificial tears add moisture without constricting anything. Matching the drop to the cause gives better results and avoids the rebound cycle entirely.