A red eye is one of the most common complaints I see, and it is also one of the most misleading: exactly the same redness can be a trivial finding that resolves by itself, or the only outward sign of a sight-threatening condition. The good news is that telling the two apart does not require medical knowledge — it requires two questions. On this page I give you exactly those two questions, and then work through each of the common causes: what it looks like, how long it lasts, and what to do about it.
The two questions that sort almost everything
Before trying to identify the precise cause, it helps to place yourself in one of three groups. Two questions do most of the work.
Does the eye hurt? Not burning, not itching, not “uncomfortable” — hurting. A dull, pressing or sharp pain is the first sign that separates harmless from urgent.
Has vision changed? Blurring that clears when you blink is tears and discharge, and that is fine. Blurring that stays, a drop in sharpness, or haloes around lights — that is not.
Those two answers produce three groups. Red and quiet — no pain, normal vision: almost always benign, and this accounts for most cases. Red and irritated — burning, itching, discharge or a gritty feeling, without real pain and without any change in vision: ocular surface conditions, annoying but not dangerous. Red and painful, or with changed vision — here you do not wait; this is the group the last part of the page was written for.
Red and quiet: the blood spot that looks terrible and is harmless
This is probably the single most common reason for an alarmed phone call. Someone wakes up, looks in the mirror, and sees a solid, vivid patch of blood on the white of the eye, sometimes covering a quarter or half of it. It looks like an emergency — and it almost never is.
The condition is called a subconjunctival haemorrhage: a tiny blood vessel beneath the conjunctiva, the transparent membrane covering the white of the eye, has broken, and the blood has spread in a thin layer underneath it. Because the conjunctiva is transparent, the blood shows at full intensity. But notice what is absent here: no pain, no drop in vision, no discharge, and no danger to the eye at all. The blood sits outside the eye itself, exactly like a bruise under the skin.
The causes are usually trivial: a hard sneeze or cough, lifting something heavy, straining, rubbing the eye, straining on the toilet, and very often — nothing identifiable at all. The haemorrhage absorbs by itself over one to three weeks, changing colour to a greenish-yellow along the way, just like a bruise. There are no drops that speed it up, and there is nothing to do but wait. Two caveats: if these haemorrhages keep recurring, it is worth checking blood pressure and clotting function, and if the haemorrhage followed a genuine blow to the eye, it needs to be examined — because then the question is what else was damaged.
The “red and quiet” group also contains simple external irritants: chlorine in a pool, smoke, dust, dry air from an air conditioner, a short night. These clear within hours to a day or two after the exposure stops, and need nothing beyond artificial tears.
Red and irritated: the ocular surface causes
This is the largest group in terms of the number of people affected, and it contains four main suspects that overlap and sometimes appear together.
Conjunctivitis is the common infectious cause. The discharge is the key: watery and tearful in viral conjunctivitis, which starts in one eye and moves to the other after two or three days; thick, purulent and yellowish in bacterial conjunctivitis, which glues the lashes together in the morning; and itch-dominant in both eyes in allergic conjunctivitis. Most resolve on their own, and most do not need antibiotics — I have covered all of that on the separate page about conjunctivitis.
Dry eye is perhaps the least recognised cause of chronic redness. The confusing sign is that excessive watering can itself be a symptom of dryness — the eye is irritated and responds by flooding. The typical picture is a red eye that worsens towards the end of the day, in front of a screen, in air conditioning or on a flight, with a gritty, burning feeling. If your redness has been recurring for months and is worse in the evening, read the page on dry eyes — there is a good chance that is the story.
Blepharitis, chronic inflammation of the eyelid margins, produces stubborn redness along the lid edge, flakes at the base of the lashes, crusting in the morning and a constant sense of irritation. It is almost always accompanied by meibomian gland dysfunction, which is why it keeps returning until the root is treated. The full account is on the page about blepharitis.
Contact lenses are a category of their own. Wearing them too long, sleeping in them, old lenses or solution that is not changed — all of these produce a red, irritated eye. Here, and only here, I ask for caution: in a lens wearer the line between irritation and keratitis is very thin, and I return to that below.
This group also contains a stye and a chalazion — a localised lump in the lid that can turn the surrounding area red — and pterygium, that triangular growth from the white of the eye towards the cornea, which is very common here because of the sun and flares up into local redness from time to time.
What does help most of this group: preservative-free artificial tears, compresses — cold for irritation and allergy, warm for blepharitis and gland problems — lid hygiene, and reducing exposure to whatever is causing the irritation. What does not help: whitening the eye with drops.
Why whitening drops are worth avoiding
Decongestant drops work by constricting the blood vessels in the conjunctiva. The result is immediate and convincing — the eye looks white within minutes. The problem is twofold. First, they treat no cause whatsoever; they conceal a sign. Second, with repeated use a rebound effect develops: the vessels dilate even more forcefully as the effect wears off, the eye becomes redder than it was to begin with, and the user instils again. This creates a cycle I see in clinic frequently, and getting out of it takes a few uncomfortable weeks. Occasional use before an event is no drama; daily use has a price.
Red and painful: do not wait
This is the small group, and it is the group this page was written for. The signs that require an examination the same day:
- Real pain in the eye — pressing, deep or sharp, as distinct from burning and itching.
- Reduced or blurred vision that does not clear on blinking.
- Strong light sensitivity, to the point of difficulty opening the eye in a lit room.
- A pupil of a different size from the other eye, or a pupil that does not react.
- Haloes around lights, headache, nausea and vomiting together with a red eye — a combination that raises the suspicion of acute glaucoma.
- A contact lens wearer with a red, painful eye.
- Injury to the eye, or a chemical splash — with a chemical, rinse under running water for ten minutes and then seek care immediately.
What lies behind these signs? Keratitis — infection of the cornea, particularly common in contact lens wearers, which can leave a permanent scar in the centre of vision. Uveitis — inflammation of the middle layer of the eye, presenting with redness around the iris, deep aching pain and strong light sensitivity, and sometimes linked to systemic autoimmune disease. Acute angle-closure glaucoma — a sudden, dramatic rise in intraocular pressure, relatively rare but a genuine emergency in which every hour counts. And a corneal abrasion or foreign body — very painful, usually healing well, but requiring confirmation that there is no infection and that the foreign body has been removed.
What these have in common: from the outside they can look exactly like conjunctivitis. The difference emerges at the slit lamp, an examination that takes minutes and immediately distinguishes an inflamed conjunctiva from an involved cornea or inner chamber. That is precisely why I do not recommend guessing at home when there is pain.
Contact lenses: the one rule worth memorising
I give this its own paragraph because it is where I see the most preventable damage. A red and painful eye in a contact lens wearer is treated as keratitis until proven otherwise. The lens impairs oxygenation and the defences of the ocular surface, allowing bacteria — and with water exposure, parasites such as acanthamoeba — to reach the cornea. The process can progress substantially within twenty-four hours.
The rule is simple and inflexible: remove the lens immediately, do not put it back in, and be examined the same day. Do not instil leftover antibiotic drops from a previous episode and wait for morning — partial treatment actually makes the diagnosis harder afterwards. And if you sleep in your lenses or swim in them, this is the moment to stop: those two habits multiply the risk severalfold.
Children and infants
In children most red eyes are viral or allergic conjunctivitis, and the rules are similar. Three situations nevertheless warrant separate mention. A baby in the first month of life with a red, discharging eye is always examined, urgently. A child with discharge that persists for weeks, without much redness, usually has a blocked tear duct rather than recurrent infection — and the solution is entirely different from another round of drops. And a child who complains of real pain, avoids light or refuses to open the eye is examined exactly like an adult with the same symptoms, meaning immediately.
What I check in clinic
The examination is short and answers three questions. First, where exactly the redness sits — diffuse redness across the conjunctiva, a ring of redness around the cornea, or deep vessels tell completely different stories. Second, whether the cornea is intact and clear: a drop of fluorescein dye reveals within seconds an abrasion, ulcer or defect invisible to the naked eye. Third, what is happening inside the eye — whether there are inflammatory cells in the anterior chamber, and what the intraocular pressure is. Those three answers sort almost every red eye into its correct place, and determine whether treatment is needed or observation will do.
If your red eye comes with pain, light sensitivity or a change in vision, if you wear contact lenses, or if the redness simply keeps returning and remains unexplained — get in touch and let’s arrange an examination at my clinic in Haifa, serving patients from across northern Israel. In many cases the examination ends in complete reassurance, and that is exactly why it is worth the time.
Frequently asked questions
Is a red eye without pain dangerous?
In the overwhelming majority of cases, no. A red eye that does not hurt, where vision is normal and there is no unusual light sensitivity, is almost always benign — a subconjunctival haemorrhage, dryness, an external irritant, or mild conjunctivitis. Pain and vision are the two signs that separate something you can watch at home from something that needs to be examined. That said, if the redness keeps coming back or lasts more than two weeks, it is worth having it checked to find out why.
What is the red patch that suddenly appeared on the white of my eye?
That is almost always a subconjunctival haemorrhage — a tiny blood vessel that broke, with the blood spreading under the clear membrane covering the white of the eye. It looks alarming, sometimes covering half the eye, but it does not hurt, does not affect vision and is not dangerous. It clears on its own within one to three weeks, changing colour to a greenish-yellow like a bruise along the way. Nothing speeds it up. If these happen repeatedly, it is worth checking blood pressure and clotting.
When does a red eye need to be seen urgently?
When there is real pain in the eye (as opposed to burning or itching), strong light sensitivity, reduced or blurred vision that does not clear when you blink, a pupil that is a different size from the other eye, nausea and vomiting together with eye pain, or when the person wears contact lenses. Each of these raises the suspicion of keratitis, uveitis or acute glaucoma — conditions where a day or two of delay changes the outcome. A red eye after an injury or a chemical splash is also seen immediately.
Are whitening eye drops a good idea?
Not as a routine. Decongestant drops constrict the blood vessels and make the eye look white, but they do not treat the cause, and with regular use they produce rebound redness: the eye becomes redder the moment you stop, which creates a cycle of dependence. They also mask a sign that should have sent you for an examination. If an eye is chronically red, it is better to find out why than to paint the problem white.
How long does a red eye take to clear?
It depends on the cause. External irritation settles within hours to a couple of days once the exposure stops. Viral conjunctivitis lasts one to three weeks. A subconjunctival haemorrhage absorbs over one to three weeks. Dry eye and blepharitis are chronic conditions that are controlled rather than cured. The practical rule: redness lasting more than two weeks without a clear explanation, or redness that keeps returning, justifies an examination — even if it does not hurt.
Why is a red eye different in a contact lens wearer?
Because in a lens wearer, a red and painful eye is treated as keratitis until proven otherwise, and that is a condition that can damage vision permanently. The lens compromises the ocular surface defences and allows bacteria — and, where water is involved, parasites — to reach the cornea. The rule is simple: a lens wearer with a red eye removes the lens immediately and is examined the same day. You do not instil drops and wait for morning.
