A bloodshot patch on one side of your eye is most often a subconjunctival hemorrhage, a small burst blood vessel just beneath the clear surface of the eye. It looks alarming but is usually painless and clears up on its own within about two weeks. Less commonly, one-sided redness can signal an infection, inflammation, or a condition that needs prompt attention, especially if pain or vision changes come with it.
Subconjunctival Hemorrhage: The Most Likely Cause
The white of your eye is covered by a thin, transparent membrane called the conjunctiva, and it’s packed with tiny blood vessels. When one of those vessels breaks, blood pools underneath and spreads across a section of the white, creating a vivid red or dark patch that’s usually confined to one area. It can look dramatic, but it rarely hurts or affects your vision at all.
This is surprisingly common. A large Taiwanese population study found that non-traumatic subconjunctival hemorrhages occur in roughly 1 out of every 167 adults per year, with the rate climbing as people age and peaking in the 60-to-69 age group. Everyday triggers include rubbing your eye too hard, a strong sneeze or cough, straining during heavy lifting, and wearing contact lenses. Blood-thinning medications also raise the risk. Sometimes there’s no identifiable cause at all.
Most subconjunctival hemorrhages resolve within two weeks without any treatment. The red patch may shift in color from bright red to yellow or green as the blood reabsorbs, similar to a bruise fading on your skin. Artificial tears can ease any mild scratchiness, but the hemorrhage itself doesn’t need medication.
Contact Lens Problems
If you wear contacts, they’re a frequent culprit behind one-sided redness. Lenses create friction against the conjunctiva, and overwear or poor hygiene amplifies that effect. Sleeping in lenses, reusing old solution, or letting microbes build up under the lens can lead to keratitis, an infection of the cornea. Symptoms include redness concentrated in one eye, blurry vision, pain, light sensitivity, and the feeling that something is stuck under your lid. Keratitis is the most common contact lens-related infection and can cause lasting damage if ignored, so redness that comes with pain or discharge in a contact lens wearer warrants a same-day eye exam.
Corneal Scratch
A corneal abrasion, a small scratch on the surface of the eye, produces redness that’s typically limited to one eye and one area. The hallmark sensation is that something is stuck in your eye even after flushing it with water. A fingernail, a piece of dust, or a contact lens edge can all cause one. An eye doctor diagnoses it by applying a fluorescein dye that makes the scratch glow under a special light. Minor abrasions heal within a few days, but deeper scratches may need antibiotic drops to prevent infection.
Episcleritis and Scleritis
When redness appears in a distinct wedge or sector of one eye and lingers for days, episcleritis or scleritis may be responsible. Both involve inflammation of the tissue layers that make up the wall of the eye, but they differ in depth and severity.
Episcleritis affects only the superficial layer. It often shows up as a pink or red patch in one section of the eye with mild discomfort or a gritty feeling. It tends to come and go and usually resolves within a couple of weeks.
Scleritis reaches deeper into the eye wall, involving both superficial and deep blood vessels. The redness is often more intense, sometimes with a purplish hue, and the pain can be severe enough to wake you at night. Scleritis is less common but more destructive if untreated, so persistent deep eye pain with redness should be evaluated promptly.
Uveitis: Inflammation Inside the Eye
Uveitis is inflammation of the inner structures of the eye, including the iris and the tissue behind it. It frequently affects just one eye at a time and can cause redness concentrated around the colored part of the eye, along with pain, light sensitivity, and blurred vision. Autoimmune conditions like inflammatory bowel disease, lupus, and sarcoidosis are linked to uveitis, though infections can also trigger it. Because untreated uveitis can permanently damage vision, any combination of redness, pain, and visual changes in one eye calls for an urgent evaluation.
Acute Angle-Closure Glaucoma
This is the rarest cause on this list but the most urgent. Acute angle-closure glaucoma happens when fluid drainage inside the eye is suddenly blocked, causing pressure to spike. It almost always strikes one eye at a time. Symptoms come on fast and include severe eye pain, a bad headache, nausea or vomiting, blurred vision, halos or colored rings around lights, and a visibly red eye. This is a medical emergency that can cause permanent vision loss within hours if the pressure isn’t relieved.
How to Tell What’s Serious
A painless red patch with no vision changes and no other symptoms is almost always a harmless burst blood vessel. You can safely watch it for a couple of weeks. If it keeps happening, mention it at your next checkup, since recurring hemorrhages can occasionally point to high blood pressure, diabetes, or a clotting issue.
Seek immediate care if any of these accompany the redness:
- Sudden vision changes, including blurriness or seeing halos around lights
- Significant eye pain or a severe headache
- Nausea or vomiting alongside eye symptoms
- Light sensitivity or fever
- Chemical or object exposure, such as a splash or debris that hit the eye
- Swelling in or around the eye, or difficulty keeping the eye open
If you’ve had recent eye surgery or an eye injection, contact your eye care provider even for mild redness. Post-procedure infections can escalate quickly and are easier to treat when caught early.