Red eye and conjunctivitis: working out the cause before you treat it

A red eye is one of the most common reasons people head straight to a pharmacy, and they usually leave with an antibiotic drop without anyone establishing what caused the redness in the first place. The problem is that redness is not a diagnosis. It is a sign that the fine blood vessels in the transparent membrane covering the white of the eye have widened — and that happens for entirely different reasons: a viral infection passed on by touch, a bacterial infection far less common than people assume, an allergic reaction to dust and pollen, chronic dryness, a foreign body, or inflammation deeper inside the eye that has nothing to do with the conjunctiva at all. A treatment that helps one of these may do nothing for another, and can delay the diagnosis of something more serious. So the first step is not choosing a drop; it is reading the signs that accompany the redness, calmly.

What the conjunctiva is, and why it reddens so easily

The conjunctiva is a thin, transparent membrane that covers the white of the eye and lines the inner surface of the lids. It is the first thing to meet air, dust, smoke and pollen, which is why it carries a dense network of fine blood vessels able to bring immune cells to any irritation within minutes. When an irritant or microbe arrives, those vessels widen to increase blood flow, and the white turns pink or red. That response is not damage in itself; it is defence. Crucially, the conjunctiva plays no part in the clarity of vision — it sits well away from the visual axis — so simple conjunctivitis should not blur your sight. When someone reports redness together with genuine blurring, the likelihood is that the problem has reached the cornea or the inside of the eye, and that distinction matters enormously in practice.

• Redness spread across the whole white and heaviest near the lids points towards conjunctivitis.
• Redness concentrated in a narrow ring around the coloured iris is a sign that calls for a specialist look, not a pharmacy drop.
• A solid, sharply bordered red patch with no pain is usually a superficial bleed under the conjunctiva — alarming to look at, generally harmless in outcome.
• Redness in one eye that appeared suddenly after work or dust deserves thought about a foreign body or a scratch before anything else.

The three common types, and how to tell them apart

Most conjunctivitis falls into three groups, and each leaves a signature you can read without instruments. Viral is by far the most common. It typically brings copious watery tearing and a gritty, sandy sensation, starts in one eye and moves to the other within two or three days, and is often preceded or accompanied by a cold, a sore throat, or a tender swollen gland in front of the ear. Bacterial is less common in adults and more frequent in children; its hallmark is a thick yellow or green discharge that glues the lids shut overnight and returns within an hour of being wiped away. Allergic separates itself from both by a single near-decisive feature: itch. Intense, persistent itching is rare in infection and central to allergy; it usually affects both eyes, comes with sneezing or a blocked nose, and recurs in particular seasons or places.

• Watery tearing, a gritty feel, and a start in one eye: most likely viral.
• Sticky purulent discharge that seals the lashes in the morning and returns quickly: most likely bacterial.
• Fierce itching in both eyes with sneezing or a dusty season: most likely allergic.
• Burning and heaviness that worsen late in the day with mild redness and no discharge: think dryness, not infection.
• Deep pain with light avoidance and blurring: not simple conjunctivitis, and it needs prompt assessment.

Why an antibiotic should not be the automatic first move

Because most cases are not bacterial to begin with. An antibacterial drop does nothing against viruses or allergy, so the redness persists and the patient concludes the case is stubborn when it is simply running its natural course. Repeated unnecessary use carries a cost: extra irritation of the ocular surface from preservatives, masking of another condition that needs diagnosing, and a contribution to antibiotic resistance more broadly. More serious still are steroid-containing drops, sometimes handed over without an examination because they calm redness quickly. Those drops can dramatically worsen certain infections and can raise eye pressure with prolonged use; they belong to a clinician who has actually examined the eye. The simple working rule: with no clear, recurring purulent discharge, starting an antibiotic is a poorly supported decision.

• Cosmetic redness-removing drops constrict vessels temporarily, then the redness rebounds harder with repeated use.
• Preservative-free artificial tears are a safe option that soothes most types without masking the diagnosis.
• A cold compress suits allergic and viral cases; a warm one is better for lid-margin problems.
• Never share a drop or a towel, and do not reuse a bottle opened during an earlier episode.

Spread at home and at work: breaking the chain

Viral conjunctivitis is contagious to a degree that surprises many, and it travels on contaminated hands more than by any other route: you touch your eye, then a door handle, a phone or a keyboard, and the virus reaches someone else hours later. That is why cases sweep through households, classrooms and offices, usually seeded by one person who has not changed how often they touch their eyes. The infectious period generally lasts as long as the heavy tearing and active redness continue, and can stretch to two weeks. Breaking the chain needs no elaborate measures, just strict adherence to simple basics during that window — and an awareness that sharing a pillow or a towel at home is the shortest path to passing it to the rest of the family.

• Frequent handwashing, especially after touching the face or wiping tears.
• A towel and pillowcase kept for the affected person throughout the symptomatic period, changed daily.
• Suspending contact lens wear entirely until redness and discharge have been gone for days, and discarding the lenses and case used during the episode.
• Postponing eye makeup and not reusing products or applicators used during the inflammation.
• Keeping a child at home during the active phase prevents an entire cycle of infection in the classroom.

When redness is a warning that cannot wait

Most conjunctivitis settles on its own, but a minority conceals a deeper problem in the cornea or inside the eye, and that group cannot afford a trial of drops and a week of waiting. The practical benchmark is that simple redness comes with no true pain and no change in visual acuity. Any redness that falls outside that description deserves specialist assessment — as does any redness in someone with particular circumstances: contact lens wear, recent eye surgery, a weakened immune system, a previous injury, or a newborn in the first weeks of life. Caution is very cheap here compared with the cost of delay, because some of these conditions can damage the cornea within a day or two.

• Reduced or blurred vision that does not clear with blinking.
• Deep pain inside the eye rather than surface burning.
• Severe light sensitivity that forces the eye shut in ordinary lighting.
• Redness while wearing contact lenses — remove the lens immediately and do not wear it again before assessment.
• Redness after a direct injury or a chemical splash — immediate irrigation with clean water comes before heading for emergency care.
• Symptoms persisting with little improvement beyond two weeks, or repeatedly recurring in the same eye.

Glasses and lenses during an episode and after it

A contact lens worn during redness is an extra surface that traps microbes and irritants against the cornea and reduces the oxygen reaching it, so the first decision in any episode is to suspend wear and switch to spectacles until symptoms have been gone for several days. After recovery, do not return to the same lens or the same case: either may carry what restarts the cycle. Sunglasses, by contrast, play a genuinely useful mechanical role — a wraparound frame cuts the dust and wind reaching the eye surface, eases the light discomfort that accompanies a flare, and makes going out in daylight far less draining. And once it is all over, one question is worth asking if episodes keep returning: is the cause really repeated infection, or is it chronic dryness, lid-margin inflammation, or uncontrolled allergy presenting each time with the same red face.