Why a red eye is often one-sided
A red eye very commonly affects one side, and on its own that is not a cause for alarm. Many of the everyday causes begin in one eye: conjunctivitis often starts on one side before spreading, a subconjunctival haemorrhage is a single patch in one eye, and a speck of grit or an irritated contact lens affects the eye it is in. Redness that affects both eyes together, by contrast, is more often the general irritation of tiredness, dryness, or an allergy.
What raises the importance of a one-sided red eye is not the side itself but what comes with it. A red eye confined to one side that is also painful, sensitive to light, or seeing less well points to a problem within that particular eye, and that is the combination worth acting on.
The redness matters less than what comes with it
The white of the eye is covered by a thin, clear membrane called the conjunctiva, which carries many fine blood vessels. When the eye is irritated or inflamed, these vessels fill with blood and the eye looks red. Because irritation often begins on one side, a red eye on its own is frequently nothing to worry about.
What tells you whether a red eye is trivial or important is the company it keeps. A red eye that is comfortable, sees normally, and is not sensitive to light is usually a minor problem. A red eye that is painful, sensitive to light, or seeing less well needs to be taken seriously.
Common causes
A subconjunctival haemorrhage looks alarming and is usually harmless. A single small vessel bleeds under the conjunctiva, producing a flat, bright red patch across part of the white of the eye. It is painless, does not affect vision, and often appears for no clear reason, or after coughing, sneezing, straining, or rubbing. It clears on its own over one to two weeks, fading like a bruise. If it happens repeatedly, it is worth having your blood pressure checked.
Conjunctivitis, inflammation of the conjunctiva, commonly begins in one eye before spreading to the other. It causes redness, a gritty feeling, and often a watery or sticky discharge. Most cases are viral or allergic and settle with simple measures, although a painful eye with marked discharge and blurred vision should be checked.
A persistently pink, gritty, tired eye is often dry eye or blepharitis, inflammation of the eyelid margins. These are common, tend to grumble on, and respond to lubrication and lid care. A speck of grit, a stray eyelash, or contact lens irritation can also redden one eye; if the sensation of something in the eye persists, or follows an injury, it should be examined.
Causes that need attention
Inflammation inside the eye, called uveitis or iritis, causes a red eye that is genuinely painful, sensitive to light, and often a little blurred. It usually affects one eye and needs prompt assessment and treatment.
Episcleritis is a common, mild inflammation of the surface tissue that reddens a sector of the eye and is usually harmless. Scleritis is different and more serious: a deep, boring, severe pain, often waking the person at night, sometimes linked to inflammatory conditions elsewhere in the body, and it needs specialist care.
The cornea, the clear front window of the eye, can become scratched, ulcerated, or infected. Infection of the cornea, keratitis, causes a red, painful, light-sensitive eye and can threaten sight, so it must be seen quickly.
- Pain, rather than mild irritation
- Sensitivity to light, or reduced or blurred vision
- Haloes around lights, headache, or nausea
- Contact lens wear
- A chemical splash, or an injury to the eye
For contact lens wearers
A red, painful eye in a contact lens wearer must never be ignored. Contact lenses raise the risk of a sight-threatening corneal infection, and the warning signs, pain, redness, light sensitivity, and blurred vision, can come on quickly. If this happens, take the lenses out, keep them and their case in case they are needed for testing, and seek same-day assessment. Do not put the lenses back in until the eye has been checked.
Comfortable and seeing well, or painful and seeing poorly
The most useful distinction is between a red eye that is comfortable and seeing normally, and a red eye that is painful or seeing poorly. A painless red eye with normal vision, such as a subconjunctival haemorrhage or mild conjunctivitis, can usually be watched and will settle. A red eye with pain, light sensitivity, or reduced vision should be assessed promptly, and a chemical splash should be rinsed with plenty of clean water immediately and treated as an emergency.
Common questions
There is a bright red patch on the white of my eye. Should I worry?
Usually not. A flat, bright red patch that is painless and does not affect vision is most often a subconjunctival haemorrhage, a small harmless bleed that clears over one to two weeks like a bruise. If it keeps happening, have your blood pressure checked. If the eye is also painful or seeing poorly, have it examined.
I have one red eye but no pain. Is it serious?
A red eye that is comfortable and seeing normally is usually a minor problem such as mild conjunctivitis, dry eye, or a small bleed, and can be watched. Redness that becomes painful, sensitive to light, or blurred should be assessed.
My eye is red and I wear contact lenses. What should I do?
Take the lenses out, keep them and the case in case they are needed for testing, and seek same-day assessment. Contact lenses raise the risk of a serious corneal infection, so a red, painful eye in a lens wearer should never be left. Do not reinsert the lenses until the eye has been checked.
When is a red eye an emergency?
Treat it as urgent if there is pain rather than mild irritation, sensitivity to light, reduced or blurred vision, haloes around lights with headache or nausea, contact lens wear, or an injury. A chemical splash should be rinsed with plenty of clean water at once and treated as an emergency.
