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The white of the eye is covered by a thin, transparent membrane, the conjunctiva. Fine blood vessels run through it and through the sclera underneath. If they widen because of irritation or inflammation, the eye turns red; doctors call this hyperaemia. A red eye is therefore not a condition in its own right but a signal — and the key question is whether only the surface is irritated or whether deeper parts of the eye are affected.
This article helps you make exactly that call: harmless, soon or immediately. The most common single cause, inflammation of the conjunctiva, is described in detail in the article on conjunctivitis; if burning and a gritty feeling are the main problem, the article on dry eyes will help.
Conjunctivitis reddens the eye over a wide area, usually more strongly towards the edge than right next to the cornea. Typical signs are a foreign body sensation, burning and discharge, but no deep pain, and vision clears when you blink. Viruses — often adenoviruses, sometimes as part of a cold — tend to cause watery discharge; the inflammation often starts in one eye and spreads to the other. Epidemic keratoconjunctivitis is highly contagious and can drag on for several weeks. Bacteria tend to cause yellowish pus and eyelids stuck together in the morning. In otherwise healthy people, bacterial conjunctivitis often clears up by itself within a few days; antibiotic drops can shorten the course somewhat. First step: wash your hands, use your own towels, do not rub your eyes. With pus, pain, worsening vision, contact lenses or symptoms lasting more than a few days, the eye needs to be seen by a doctor.
In allergic conjunctivitis, itching is the key symptom. Both eyes are red and watery, the eyelids often swell, and sneezing and a runny nose are added — typical of hay fever (allergic rhinitis) in the pollen season, or all year round with a dust mite or pet allergy. Rubbing fuels the cycle of itching and redness. First step: avoid the allergen, use cool compresses and anti-allergy eye drops from the pharmacy; if the symptoms are severe, have the allergy assessed by a doctor. If the eyelids are swollen too, the article on swollen eyelids will help you make sense of it.
If the tear film is unstable, the surface of the eye is constantly slightly irritated. The eyes are then mildly red, burn and feel gritty — worse at a screen, in heated or air-conditioned air and towards the evening. Often there is a chronic inflammation of the eyelid margins (blepharitis) with blocked oil glands behind it; the eyelid margins then look red and crusty. Rosacea can also affect the eyes, and Sjögren's syndrome also dries out the mouth. First step: screen breaks, preservative-free artificial tears and eyelid care with warmth.
If a small vessel under the conjunctiva bursts, a sharply defined, bright red patch appears in the white of the eye (subconjunctival haemorrhage). It does not hurt, your vision is unchanged, and often you only notice it when you look in the mirror or someone else points it out. Common triggers are hard coughing, sneezing, straining, vomiting or rubbing; sometimes there is no cause at all. Blood-thinning medicines and high blood pressure make such bleeds more likely. The blood is usually broken down within one to three weeks and turns yellowish as it does. First step: wait. If you have repeated bleeds, take anticoagulants or are unsure about your blood pressure, have your blood pressure and, if necessary, your clotting checked at your GP practice. This does not apply after a blow to the eye or if there is pain — then the eye needs to be seen by an eye specialist.
In episcleritis, the layer of tissue directly above the sclera becomes inflamed. Usually a localised, pinkish-red area is visible; the eye feels a little pressured but hardly hurts, and vision stays good. It often affects young adults, comes and goes, and frequently clears up by itself. Scleritis, an inflammation of the sclera itself, is quite different: the eye is deep dark red to bluish-purple and has a boring pain, often spreading to the forehead and jaw, and the pain wakes you at night. It is more common with autoimmune diseases such as rheumatoid arthritis and needs prompt treatment by an eye specialist and usually also by an internal medicine or rheumatology specialist.
In an inflammation of the iris (iritis), the redness typically sits in a ring around the cornea (ciliary injection). It comes with dull eye pain, marked sensitivity to light, blurred vision and often a smaller pupil on the affected side. Usually only one eye is affected. Iritis is more common together with ankylosing spondylitis, inflammatory bowel diseases such as Crohn's disease, psoriasis with joint involvement, or sarcoidosis. First step: see an eye specialist the same day. Untreated, it can lead to adhesions in the pupil and raised eye pressure.
In acute angle closure, the outflow of the fluid in the front of the eye (aqueous humour) is suddenly blocked, and the pressure in the eye rises sharply within a short time. The eye is deep red, the cornea looks dull, and the pupil is mid-dilated and barely reacts to light. Severe eye pain and headache, blurred vision with coloured rings around lights, and nausea and vomiting are added — sometimes so severe that the stomach is suspected first. Those most at risk are long-sighted people from middle age with a shallow front chamber of the eye. Triggers can be darkness, stress, pupil-dilating drops or certain medicines (see the medication angle). More on the condition under glaucoma. First step: go to an eye clinic or emergency department immediately — without treatment, the optic nerve can be permanently damaged within hours.
A scratch on the cornea (erosion), a foreign body or an inflammation of the cornea (keratitis) cause a red eye with real pain, sensitivity to light, tears and the urge to squeeze the eye shut. With contact lenses, keratitis caused by bacteria or amoebae is particularly feared because it can quickly cost you your sight — what to watch out for is explained in the guide Medications and contact lenses. Herpes viruses can infect the cornea, often repeatedly in the same eye; in people prone to herpes, this is a known pattern. Blisters on the forehead, upper eyelid or tip of the nose together with a red eye point to shingles around the eye. First step: in all these cases, have the eye examined by an eye specialist the same day; take contact lenses out immediately.
These observations do not replace a diagnosis or an examination with a slit lamp. But they help you judge how urgent it is and report precisely at your appointment.
Prostaglandin drops such as latanoprost reliably lower eye pressure, but according to the summary of product characteristics they very commonly redden the conjunctiva, especially in the first few weeks. Brimonidine can trigger allergic reactions of the conjunctiva, some of which only appear after longer use. The preservative benzalkonium chloride additionally irritates the surface of the eye when used for years. Preservative-free alternatives or other active substances are often available. But never stop glaucoma drops on your own initiative, because the pressure in the eye then rises without you noticing.
Vasoconstricting eye drops with active substances such as tetryzoline or naphazoline make red eyes look white for a few hours. With longer use, the vessels widen all the more once the effect wears off — the eye becomes redder, and a cycle develops similar to that with decongestant nasal sprays (see nasal spray dependence). They are therefore only suitable for a few days. They must not be used if you have narrow-angle glaucoma.
Anticoagulants such as apixaban, rivaroxaban or phenprocoumon, as well as antiplatelet medicines such as aspirin (ASA), make bleeds under the conjunctiva more likely. A single subconjunctival haemorrhage is no reason to interrupt blood thinning. If the red patches become more frequent or other signs of bleeding such as nosebleeds or bleeding gums appear, talk to your practice; with phenprocoumon, the INR value should then be checked. More in the guide Living with blood thinners.
In people with a shallow front chamber of the eye, active substances that dilate the pupil can promote angle closure: medicines with anticholinergic effects such as amitriptyline, bladder medicines such as oxybutynin, antispasmodics such as butylscopolamine or older antihistamines, and also vasoconstricting active substances. The guide on anticholinergic burden gives an overview of such medicines. Topiramate is a special case: according to the summary of product characteristics, it can trigger sudden short-sightedness with secondary angle-closure glaucoma — usually in the first month of treatment and often in both eyes. Red, painful eyes with blurred vision after starting such a treatment are an emergency.
Isotretinoin dries out the eyes and eyelid margins and can promote conjunctivitis and blepharitis. According to its summary of product characteristics, the biologic dupilumab for atopic dermatitis commonly causes conjunctivitis. Bisphosphonates such as alendronic acid have rarely been linked to inflammation of the iris or sclera. If redness fits in time with a new active substance, it is worth asking your doctor or pharmacist — and checking the interaction check if several medicines are combined.
A painful red eye is not a case for prolonged self-treatment. Without medical advice, you should usually take over-the-counter painkillers such as ibuprofen or paracetamol for no longer than three to four days — and they can mask a serious cause. There is a good reason why anaesthetic eye drops are not available for self-treatment: used repeatedly, they damage the cornea. More in the painkiller comparison.
Doses and symptoms over time make the timing visible for your doctor's appointment.
Glaucoma drops, artificial tears and tablets in one place — with dose reminders and an interaction check.
This article is for general information and does not replace medical advice, diagnosis or treatment. The self-tests described — the vision test, the light check and the pupil comparison — are a guide and not a diagnosis. If you have a suddenly red, painful eye with worsening vision, a chemical burn or an injury, pain while wearing contact lenses or after eye surgery, please contact a doctor or the emergency services without delay.
