Red eyes happen when tiny blood vessels on the surface of your eye dilate and fill with more blood than usual, making the white of your eye look pink or red. The causes range from completely harmless (a night of poor sleep) to genuinely urgent (acute glaucoma), so the real question is what other symptoms come with the redness.
Why Eyes Turn Red
The white of your eye is covered by a thin, clear membrane called the conjunctiva, which contains dozens of small blood vessels. Normally these vessels are so narrow they’re nearly invisible. When something irritates, inflames, or injures your eye, your body releases chemical signals that force those vessels open wider. More blood flows to the surface, and the white of your eye turns visibly red. This process is the same basic inflammation response that makes a scraped knee look red and swollen.
The Most Common Causes
Allergies
Seasonal allergies are one of the most frequent reasons for red eyes, and they typically affect both eyes at the same time. You’ll usually notice itching, watering, and puffiness alongside the redness. If your red eyes show up during pollen season or after being around dust, pet dander, or mold, allergies are the likely culprit.
Pink Eye (Conjunctivitis)
Pink eye can be caused by a virus, bacteria, or an allergen. Viral conjunctivitis usually starts in one eye and spreads to the other within a day or two, producing watery discharge. Bacterial conjunctivitis tends to cause thicker, yellowish or greenish discharge that can crust your eyelids shut overnight. Both viral and bacterial forms are contagious. Allergic conjunctivitis isn’t contagious and almost always affects both eyes with significant itching.
Dry Eyes
When your eyes don’t produce enough tears or the tears evaporate too quickly, the surface of the eye becomes irritated and inflamed. Dry eye redness usually affects both eyes and gets worse throughout the day, especially if you spend long hours looking at screens or in air-conditioned rooms. Left untreated over time, chronic dry eye can damage the corneal surface.
Broken Blood Vessel
A subconjunctival hemorrhage looks alarming: a bright red patch that covers part or all of the white of your eye. Despite its dramatic appearance, it’s painless, doesn’t affect your vision, and is almost always harmless. It happens when a tiny blood vessel breaks, often from sneezing, coughing, straining, rubbing your eye too hard, or even lifting something heavy. The blood gets trapped under the conjunctiva, so it can’t be blinked or wiped away. Most cases resolve on their own within about two weeks, sometimes turning yellowish as the blood clears. People on blood thinners are more prone to these.
Contact Lenses and Eye Drops
Wearing contacts too long, sleeping in them, or using a dirty pair can irritate or even infect the cornea. Contact lens use is actually considered a red flag by eye care professionals evaluating a red eye, because it raises the risk of corneal ulcers and keratitis (corneal inflammation), both of which can threaten vision.
Ironically, over-the-counter redness-relieving eye drops can make things worse over time. Traditional drops work by squeezing blood vessels shut, but with repeated use your eyes can become dependent on them. When the drops wear off, the vessels bounce back even wider than before, creating a cycle of worsening redness. If you’ve been using redness-relief drops regularly and your eyes seem redder than ever, the drops themselves may be the problem.
Irritants and Foreign Objects
Smoke, chlorine, wind, dust, and getting a small particle stuck under your eyelid can all trigger redness. This type usually clears up once the irritant is removed or your eye finishes flushing it out.
Causes That Need Quick Attention
Acute Angle-Closure Glaucoma
This is a medical emergency. It happens when fluid pressure inside the eye spikes suddenly because the drainage system gets blocked. The symptoms are hard to miss: severe eye pain, a red eye, headache, nausea or vomiting, blurred vision, and seeing rainbow-colored halos around lights. Without prompt treatment, it can cause permanent vision loss. If you experience this combination of symptoms, go to an emergency room.
Uveitis
Uveitis is inflammation of the middle layer of the eye wall, which includes the iris and surrounding structures. It causes eye redness along with pain, light sensitivity, blurred vision, and sometimes dark floating spots. It can be triggered by infections, autoimmune conditions, or eye injuries, and it requires treatment to prevent complications like vision loss.
Corneal Ulcer and Keratitis
An open sore on the cornea or inflammation of the cornea causes redness accompanied by significant pain, tearing, light sensitivity, and sometimes a feeling that something is stuck in your eye. Contact lens wearers are at higher risk. These conditions can scar the cornea and permanently affect vision if not treated.
Scleritis
Scleritis is a deeper inflammation of the tough white outer wall of the eye itself, not just the surface membrane. It causes a boring, intense pain that can wake you from sleep and often gets worse with eye movement. It’s frequently associated with autoimmune conditions like rheumatoid arthritis and needs treatment to prevent structural damage to the eye.
How to Tell Harmless From Serious
A simple rule guides how eye care professionals triage red eyes: a painless red eye with normal vision rarely needs urgent evaluation. A painful red eye with any vision changes almost always does. Here’s what to look for.
Signs that suggest something routine:
- Both eyes are red, especially with itching, tearing, or allergy symptoms
- No pain, just mild irritation or grittiness
- Vision is completely normal
- A bright red patch with no pain and no vision change (likely a broken blood vessel)
Signs that call for prompt medical care:
- Sudden vision changes in the affected eye
- Severe pain, not just irritation
- Intense light sensitivity
- Nausea or vomiting with eye pain
- Halos around lights
- A chemical splash or penetrating injury
- Swelling in or around the eye, especially with fever
- Inability to open or keep the eye open
One-sided redness is generally more concerning than redness affecting both eyes. And if you wear contact lenses and develop a red, painful eye, take the lens out immediately and get evaluated, because the risk of a corneal infection is real.
Managing Mild Red Eyes at Home
For redness caused by allergies, dryness, or minor irritation, a few straightforward strategies help. Cool compresses over closed eyes reduce swelling and soothe irritation. Preservative-free artificial tears (not redness-relief drops) add moisture without the rebound effect. If allergies are the trigger, over-the-counter antihistamine eye drops can calm itching and redness within minutes.
Avoid rubbing your eyes, even when they itch. Rubbing increases inflammation, can introduce bacteria, and in the case of a broken blood vessel, may make the bleeding worse. If you suspect a foreign object is trapped under your eyelid, try flushing with clean water or saline rather than digging at it with your fingers.
Redness from a broken blood vessel, fatigue, or mild irritation typically clears within a few days to two weeks without any treatment at all.