You look at yourself in the mirror in the morning and the white of the eye is no longer white: a bright red patch, sharp-edged, has appeared out of nowhere. Or a diffuse redness, with stuck eyelids and the feeling of grit under the lashes. Usually a red eye is a benign complaint, which the family doctor can manage without difficulty1.
The causes of red eye, however, are many, and a small proportion threaten vision. The difference lies not so much in how red the eye is, but in what accompanies it: pain, discomfort in light, changing vision3.
In brief
- The most common cause of red eye is conjunctivitis, which may be viral, bacterial or allergic1.
- A bright red patch on the white of the eye is often a small burst blood vessel; if the eye does not hurt and vision does not change, it is usually nothing serious2.
- Pain, severe discomfort in light or changing vision indicate that a more serious cause must be sought3.
- People who wear contact lenses and have a red eye need to be seen soon: it can be an infection of the cornea2,7.
- A red, very painful eye with coloured halos around lights, headache and nausea may be an acute glaucoma attack: call 112 or go to an emergency department (A&E)9.
In this article
Why the eye turns red
Redness is the main sign of inflammation of the eye1: the small blood vessels on the surface dilate and become visible. For the examiner, where the red is concentrated also matters. If it is diffuse, mainly over the conjunctiva, the transparent membrane covering the white of the eye, the problem usually concerns the surface. If it forms a ring around the cornea, the transparent part in front of the iris, it may indicate inflammation of the cornea, of the iris or of the ciliary body, the structure behind the iris4.
It is not a distinction you can make reliably by yourself in the mirror. It does, however, help to understand why, faced with the same red eye, the doctor asks precise questions and looks at the eye carefully.
The most common causes of red eye
A review for family doctors identifies conjunctivitis as the most frequent cause, and among the other common causes lists blepharitis (inflammation of the eyelid margin), corneal abrasions, foreign bodies, subconjunctival haemorrhage (the ‘burst capillary’), keratitis, iritis, glaucoma, chemical burns and scleritis1.
The UK National Health Service (NHS) links the most common causes to what you notice2:
| What you notice | Possible cause |
|---|---|
| A bright red area on the white of the eye | A burst blood vessel |
| A feeling of grit or burning, sticky eyes | Conjunctivitis |
| Irritated, blurred or watering eyes | Dry eye |
| Eyelids that itch, are irritated, reddened or crusted | Blepharitis |
| A feeling that there is something in the eye | An eyelash growing inwards |
| Swollen or drooping eyelid, or one with a lump | A problem with the eyelid |
If the eye does not hurt, vision has not changed and you do not feel unwell, according to the NHS it is probably nothing serious and may get better on its own in a few days; in the meantime its advice is not to touch or rub the eye and not to use contact lenses2.
Conjunctivitis
Conjunctivitis may be viral, bacterial or allergic, or due to irritants1,5. Telling viral from bacterial is difficult even for the doctor, because there is no simple test that does it with certainty1. Viral and bacterial forms tend to resolve on their own and serious complications are rare1, but symptoms can last up to three weeks5.
The study
Antibiotics or not? The Cochrane review
A Cochrane review updated in 2023 pooled 21 randomised trials with 8,805 people with bacterial conjunctivitis, comparing antibiotic eye drops or ointments with a placebo. In 5 of these trials, with 1,474 people, symptoms had resolved between the fourth and the ninth day in 55.5% of those who had received the placebo and in 68.2% of those using an antibiotic: about 56 people in 100 without antibiotic and 68 in 100 with the antibiotic. Antibiotics also reduced the probability that the infection would persist, and no serious undesirable effects were reported, although the evidence on this point was of very low certainty. The authors speak of a modest benefit6.
What to do in an individual case is decided by the doctor, who assesses the symptoms and their duration. We describe how the different types are told apart in Viral, bacterial or allergic conjunctivitis.
The burst capillary
A bright red patch on the white of the eye usually corresponds to a small burst blood vessel2. It looks alarming, but if there is no pain and vision is normal it falls among the cases the NHS usually regards as not serious; an eye that is a very dark red, or redness after an injury, must instead be assessed urgently2.
When red eye is not trivial
A review for emergency department doctors sums up the criterion in one sentence: a red eye without discomfort in light, pain or visual disturbance is most often conjunctivitis; the presence of any of these symptoms indicates the need to look for a more serious cause3.
The most serious causes are keratitis, an inflammation or infection of the cornea; iritis, an inflammation of the iris that is part of the uveitides; scleritis; acute glaucoma; chemical burns1. Among the reasons for a rapid referral to the ophthalmologist, the same review for family doctors lists severe pain, loss of vision, abundant purulent discharge, involvement of the cornea, an injury, a recent eye operation, a distorted pupil, herpes infection and recurrent infections1. We look more closely at inflammation inside the eye in our article on uveitis.
Acute glaucoma deserves a separate mention. According to the NHS it presents with intense eye pain, red eye, tenderness around the eye, coloured rings around lights, hazy vision, nausea or vomiting and headache, and is a medical emergency9. We describe it in our article on open-angle and angle-closure glaucoma.
Red eye and contact lenses
A person who wears contact lenses and has a red eye falls, for the NHS, among the cases to be seen soon, because it may be an infection2. Infection of the cornea, microbial keratitis, is rare but can seriously threaten vision; in lens wearers it is more frequent than in non-wearers and is more often caused by bacteria such as Pseudomonas and by an amoeba, Acanthamoeba7.
Its frequency varies with the type of lens and the way it is used7: a 2021 review, for its calculations, considered a range between 1 and 25 cases per 10,000 lens wearers a year8. The most important risk factors, according to a 2020 review, are sleeping in lenses, not washing and drying your hands before touching them and poor case hygiene; the lowest risk of serious disease is seen with daily disposable lenses and with rigid gas-permeable lenses7. We discuss this in Sleeping in contact lenses.
When to talk to an ophthalmologist
- Call 112 or go to an emergency department (A&E) if you have a red eye together with: changes in vision (wavy lines, flashes, loss of vision), pain when looking at light, severe headache with nausea, an eye that is a very dark red, one pupil larger than the other, an injury or an object that has gone into the eye, a chemical substance2. A red, very painful eye with coloured halos around lights may be an acute glaucoma attack9. The full list is on our page about eye emergencies.
- The same day, from the doctor or the ophthalmologist: a very red and painful eye, a red eye in someone who wears contact lenses, a red eye in a newborn baby under 28 days old2.
- In the following days: a red eye in a child under 2 years old, a red eye with sticky discharge, redness or eyelid problems that last more than a few days, or if something worries you2.
Frequently asked questions
Red eye without pain: when to worry?
If the eye does not hurt, vision is normal and you do not feel unwell, it is usually nothing serious and improves in a few days2. It becomes a reason to be seen if pain, discomfort in light or changes in vision appear, if there is sticky discharge or if the redness lasts more than a few days2,3.
Does conjunctivitis go away on its own?
Often it does: viral and bacterial forms tend to resolve on their own, although symptoms can last up to three weeks1,5. In the Cochrane review, without antibiotic about 56 people in 100 with bacterial conjunctivitis were better within 4–9 days6.
I have a red eye and wear contact lenses: what should I do?
The NHS advises not using the lenses until the eye gets better and asking for a visit soon, because it may be an infection of the cornea2,7.
In red eye, the colour matters less and what accompanies it matters more. Redness without pain and with normal vision is almost always a passing nuisance; pain, discomfort in light, changing vision or contact lenses shift the picture towards a rapid visit2,3.
Sources
- Cronau H, Kankanala RR, Mauger T. Diagnosis and management of red eye in primary care. Am Fam Physician. 2010;81(2):137-44. PubMed 20082509
- NHS. Red eye. nhs.uk
- Bertolini J, Pelucio M. The red eye. Emerg Med Clin North Am. 1995;13(3):561-79. PubMed 7635083
- Roy H. The red eye. Compr Ther. 2006;32(1):43-6. doi:10.1385/comp:32:1:43 · PubMed 16785582
- Gin C, Crock C, Wells K. Conjunctivitis: A review. Aust J Gen Pract. 2024;53(11):847-852. doi:10.31128/AJGP-09-23-6960 · PubMed 39499843
- Chen YY, Liu SH, Nurmatov U, van Schayck OC, Kuo IC. Antibiotics versus placebo for acute bacterial conjunctivitis. Cochrane Database Syst Rev. 2023;3(3):CD001211. doi:10.1002/14651858.CD001211.pub4 · PubMed 36912752 · PMC
- Stapleton F. Contact lens-related corneal infection in Australia. Clin Exp Optom. 2020;103(4):408-417. doi:10.1111/cxo.13082 · PubMed 32363626
- Bullimore MA, Ritchey ER, Shah S, Leveziel N, Bourne RRA, Flitcroft DI. The Risks and Benefits of Myopia Control. Ophthalmology. 2021;128(11):1561-1579. doi:10.1016/j.ophtha.2021.04.032 · PubMed 33961969
- NHS. Glaucoma. nhs.uk
Study titles are given in their original language of publication. The superscript numbers in the text refer to these sources; citation details come from PubMed (National Library of Medicine).
