Red eye happens when tiny blood vessels on the surface of your eye widen and fill with more blood than usual, making the white of your eye look pink or red. This process, called conjunctival hyperemia, is your body’s response to irritation, infection, allergies, dryness, or injury. Most causes are minor and clear up on their own, but a few need prompt attention.
What Happens Inside Your Eye
The white part of your eye is covered by a thin, transparent membrane called the conjunctiva. It’s laced with tiny blood vessels that are normally too small to see. When something irritates or inflames the area, your body releases molecules like histamine and other inflammatory signals that force these vessels to dilate. The wider vessels carry more blood to the surface, which is why the white of your eye turns red. This response is actually part of your immune system at work, delivering infection-fighting cells and protective proteins to the irritated tissue.
Allergies
Pollen, pet dander, dust mites, and mold are among the most common triggers for red, itchy eyes. When an allergen lands on your conjunctiva, your body releases histamine, which swells the blood vessels almost immediately. Both eyes are usually affected, and the hallmark symptom is intense itching along with watering and puffiness. Seasonal allergies tend to flare in spring and fall, while year-round allergens like dust and pet hair can keep your eyes pink indefinitely if the exposure continues.
Over-the-counter antihistamine eye drops or oral antihistamines typically bring relief within 20 to 30 minutes. Some drops work by stabilizing mast cells, the white blood cells responsible for releasing histamine in the first place, which helps prevent the reaction rather than just treating it after the fact.
Infections: Viral and Bacterial
Viral conjunctivitis (classic “pink eye”) is highly contagious and usually starts in one eye before spreading to the other within a day or two. It causes a watery discharge and a gritty, irritated feeling. There’s no antibiotic that speeds it up. It runs its course in one to three weeks.
Bacterial conjunctivitis produces thicker, yellow-green discharge that can crust your eyelids shut overnight. It responds to antibiotic eye drops, and most people improve within a few days of starting treatment. Both types spread easily through touching your eyes and then touching surfaces, sharing towels, or close contact with others.
Dry Eyes and Screen Time
You normally blink about 15 times per minute. When you stare at a screen, that rate drops by roughly half. Fewer blinks mean your tear film evaporates faster, leaving the surface of your eye exposed and irritated. The result is redness, a burning or sandy sensation, and sometimes blurry vision that clears when you blink hard. This is one of the most common reasons people develop chronic low-grade redness, especially if they spend hours a day on computers or phones.
The 20-20-20 rule helps: every 20 minutes, look at something 20 feet away for 20 seconds. Artificial tears (the preservative-free kind, if you use them frequently) add moisture back. Positioning your screen slightly below eye level also reduces how much of your eye surface is exposed to air.
Contact Lens Problems
Contact lenses are a major source of red eye, especially when worn too long, slept in, or not cleaned properly. The CDC lists several specific complications tied to lens wear: corneal abrasions (tiny scratches on the eye’s surface), giant papillary conjunctivitis (irritated bumps under the eyelid from chronic lens friction), and a condition called contact lens-induced acute red eye, or CLARE, which causes sudden redness and irritation. Over time, lenses that limit oxygen flow to the cornea can even trigger new blood vessel growth across the eye’s surface, creating persistent redness.
If your eyes are consistently red after wearing contacts, the lens fit, material, or your wearing schedule may need to change. Sleeping in lenses that aren’t designed for overnight wear dramatically increases infection risk.
Burst Blood Vessels
Sometimes a single blood vessel breaks, causing a bright red patch on the white of your eye. This is a subconjunctival hemorrhage, and it looks alarming but is almost always harmless. Common triggers include coughing, sneezing, straining (during heavy lifting or constipation), vomiting, and rubbing your eye too hard. Blood thinners and high blood pressure can make it more likely.
The red spot doesn’t hurt, doesn’t affect your vision, and clears on its own in one to three weeks as the blood gradually reabsorbs. No treatment is needed.
Deeper Inflammation
Not all red eye is on the surface. Episcleritis inflames the layer just beneath the conjunctiva, causing a localized patch of redness that may be slightly tender but usually isn’t serious. It often resolves in a week or two.
Uveitis is more concerning. It affects structures inside the eye and causes deep, aching pain, light sensitivity, and sometimes blurred vision. It can threaten your sight if untreated. Scleritis, inflammation of the tough outer wall of the eye, also causes severe pain that can radiate to the forehead or jaw. Both conditions need evaluation by an eye specialist, not just over-the-counter drops.
Other Common Triggers
- Chlorine and swimming: Pool chemicals strip away your tear film, leaving eyes red and irritated for hours afterward. Goggles prevent it entirely.
- Smoke and air pollution: Particulates and chemical irritants in the air trigger the same inflammatory response as allergens, dilating blood vessels on contact.
- Alcohol: Drinking causes blood vessels throughout your body to relax and widen, including those in your eyes. Heavy drinking commonly produces noticeable redness.
- Lack of sleep: Your eyes need closed-lid rest to stay lubricated and recover from the day’s exposure. Skipping sleep leaves them dry and bloodshot.
- Cannabis: THC lowers blood pressure and dilates blood vessels, including those in the conjunctiva. The classic “stoner red eye” is a direct pharmacological effect, not from smoke irritation alone.
When Red Eye Needs Urgent Attention
Most red eye is temporary and harmless. But certain symptoms signal something that could threaten your vision. These include severe pain that doesn’t improve, any noticeable loss of vision, a pupil that looks irregular or doesn’t react to light normally, copious thick discharge, or redness following an eye injury or recent eye surgery. A history of herpes infection affecting the eye also warrants immediate care, since recurrence can scar the cornea.
If your red eye is mild, not painful, and your vision is normal, it’s reasonable to try lubricating drops and wait a day or two. If it persists beyond a week, worsens, or comes with pain or vision changes, that’s when professional evaluation matters most.