Red eye: what are the causes and when should you worry?
An eye that turns red overnight is worrying, and understandably so. Most of the time, the cause is harmless and resolves well. But a red eye can also be the sign of a more serious condition, especially if it comes with pain, a drop in vision or if you wear contact lenses. Knowing how to tell the situations that can wait from those that call for seeing an ophthalmologist the same day is what matters most. Here is a clear guide to help you react at the right moment.
The short answer: often harmless, sometimes urgent
Key point. A red eye that is painless and comes with no drop in vision most often reflects a harmless cause: conjunctivitis, dry eye, a small haemorrhage under the conjunctiva or a passing irritation. Some signs, however, should raise concern and prompt you to see an ophthalmologist the same day or, failing that, go to an eye emergency department: significant pain, a drop in vision, marked sensitivity to light or the sensation of a foreign body that will not clear. In a contact lens wearer, a red eye means removing the lens and seeking care promptly. In these cases, the aim is to rule out damage to the cornea, an acute glaucoma attack or internal inflammation of the eye. When in doubt, an ophthalmology opinion settles the matter.
The most common and harmless causes
The most common cause is conjunctivitis, whether viral, bacterial or allergic, which produces a red eye with discharge or itching, but without true pain or a drop in vision. Next comes dry eye, very common with screen use, which irritates the surface and makes it turn red. A subconjunctival haemorrhage looks alarming: a bright red patch appears all at once on the white of the eye, often after straining, a cough or for no obvious reason. It is painless, does not affect vision and clears on its own within one to two weeks, like a bruise. Finally, blepharitis, meaning inflammation of the eyelid margin, can maintain a chronic redness. These causes are most often harmless.
Bacterial conjunctivitis often clears up on its own. A 2023 Cochrane review (Chen), covering 21 trials and 8,805 participants, found that 55.5% of patients on placebo were cured between day 4 and day 9, compared with 68.2% on antibiotic eye drops: the antibiotic modestly speeds up recovery. Viral conjunctivitis, for its part, does not respond to antibiotics; a 2025 review (Shunmugam) points out that supportive care and hygiene remain central to its management, and that local treatment may be considered in severe forms. A 2022 Cochrane review (Liu) on epidemic keratoconjunctivitis, the most contagious form, found only small trials at high or unclear risk of bias.
A subconjunctival haemorrhage, that painless bright red patch, is most often harmless. In a Japanese series (Mimura, 2010), an injury or a contact lens was the main cause in younger patients, whereas 47.5% of patients over 60 had high blood pressure. A 2013 review (Tarlan) recommends a check-up, in particular of blood pressure and blood clotting, when it recurs or persists.
The signs that call for a consultation the same day
Red eye: watch or see a doctor?
It is not how red the eye is that matters, but pain and vision.
- No real pain
- Normal vision
- Discharge or itching
- Bright red patch on the white of the eye
- Deep or intense pain
- Drop in vision
- Marked discomfort in light
- Coloured halos with headache
- Blow to the eye
Educational diagram. If no ophthalmologist is available the same day, an eye emergency department takes over.
- Genuine pain: not mere discomfort, but deep or intense pain. It points to damage to the cornea, internal inflammation or an acute glaucoma attack.
- A drop in vision: any decline in eyesight together with a red eye should prompt seeing an ophthalmologist the same day or, failing that, going to an eye emergency department.
- Strong sensitivity to light: being dazzled or in pain in bright light points to an inflammation that should be taken seriously.
- Wearing contact lenses: a red and painful eye in a lens wearer raises the fear of a corneal abscess. The lens should be removed and care sought promptly.
- Coloured halos and headaches, or a recent injury: these situations also call for seeing an ophthalmologist the same day or, failing that, going to an eye emergency department.
Contact lenses expose the eye to an infection of the cornea, keratitis. In a large Australian study (Stapleton, 2008), it occurred each year in 1.9 out of 10,000 soft lens wearers using daily wear, but in 19.5 out of 10,000 when the lenses were worn overnight; vision loss occurred in 0.6 out of 10,000 wearers, all types of wear combined. A case-control study on the Acanthamoeba amoeba (Carnt, 2023) also links the risk to showering with lenses in, reusing daily disposable lenses and sleeping in them.
An acute angle-closure glaucoma attack is the other emergency not to be missed. A 2019 review (Flores-Sánchez) points out that it should be suspected in a red eye of sudden onset with a dilated pupil, a dull pain and headaches; it mainly affects predisposed eyes, such as those of long-sighted people. Some medicines can trigger it in people who are unaware of their predisposition (Ah-Kee, 2015). It requires seeing an ophthalmologist the same day or, failing that, going to an eye emergency department.
How does the ophthalmologist sort it out?
A few simple questions already guide the diagnosis a great deal: how long the eye has been red, one eye or both, whether there is pain and how intense, whether vision has changed. The slit-lamp examination then allows the cornea and the clarity of the eye to be observed and, if needed, the pressure to be measured. A few minutes are often enough to tell a routine conjunctivitis from a more serious condition. That is precisely the value of seeking care when in doubt: if the red eye is harmless, you will be reassured, and a true emergency can be managed in time, which can make all the difference for the cornea and for vision.
Red eye in figures
Three figures behind the right reflexes.
These figures describe studies, not an individual case.
FAQ
Frequently asked questions
I have a bright red patch on the white of my eye, is it serious?
Most often no. This is a subconjunctival haemorrhage, a small vessel that has bled under the surface. If the eye does not hurt and vision is normal, it clears on its own within one to two weeks. If it recurs often, a blood pressure check may be worthwhile.
Is a red eye contagious?
It depends on the cause. Viral or bacterial conjunctivitis can be contagious, which is why it is important to wash your hands often and not to share towels or make-up. Dry eye or a subconjunctival haemorrhage are not.
I wear contact lenses and my eye is red, what should I do?
Remove the lens straight away and do not put it back in. If the eye is painful, sensitive to light or if vision drops, seek urgent care. In a lens wearer, a painful red eye should prompt ruling out a corneal abscess.
Can I use eye drops bought at the pharmacy?
Artificial tears relieve simple irritation or dryness. However, it is better to avoid self-medicating with eye drops containing corticosteroids without medical advice, as they can worsen certain causes. If there is pain or a drop in vision, seek care rather than adding more drops.
Red eye with pain: when should you be seen quickly?
Seek care quickly if the redness comes with genuine pain, meaning deep or intense rather than simple irritation, with a drop in vision, marked sensitivity to light, coloured halos around lights together with headache, or if it follows a recent injury. These situations prompt a search for damage to the cornea, an acute glaucoma attack or internal inflammation of the eye. If the pain is severe or vision drops suddenly, go straight to an ophthalmology emergency service rather than waiting for an appointment.
My eye has been red for several weeks: what can that be?
Redness that settles in over time, without pain or loss of vision, points towards chronic irritation rather than an emergency. Two causes come up often: dry eye, very common with screen use, which irritates the surface of the eye and makes it red, and blepharitis, an inflammation of the eyelid margins that keeps the redness going. Neither is serious, but an examination identifies them precisely and allows suitable treatment rather than simply putting up with them.
How does the ophthalmologist work out the cause of a red eye?
A few simple questions already narrow things down a great deal: how long the eye has been red, one eye or both, whether there is pain and how intense, whether vision has changed. Examination with the slit lamp then allows the cornea and the clarity of the eye to be inspected, and the pressure measured if necessary. A few minutes are often enough to tell an ordinary conjunctivitis from something more serious: that is the value of being seen when in doubt.
Scientific sources
- Frings A, Geerling G, Schargus M. Red Eye: A Guide for Non-specialists. Dtsch Arztebl Int. 2017;114(17):302-312. PMID 28530180.
- Winters S, Frazier W, Winters J. Conjunctivitis: Diagnosis and Management. Am Fam Physician. 2024;110(2):134-144. PMID 39172671.
- Chen YY, et al. Antibiotics versus placebo for acute bacterial conjunctivitis. Cochrane Database Syst Rev. 2023. PMID 36912752.
- Shunmugam M, et al. An Update on Viral Conjunctivitis Treatment Strategies: A Narrative Literature Review. Microorganisms. 2025. PMID 40871219.
- Liu SH, et al. Topical pharmacologic interventions versus placebo for epidemic keratoconjunctivitis. Cochrane Database Syst Rev. 2022. PMID 35238405.
- Mimura T, et al. Recent causes of subconjunctival hemorrhage. Ophthalmologica. 2010. PMID 19738393.
- Tarlan B, et al. Subconjunctival hemorrhage: risk factors and potential indicators. Clin Ophthalmol. 2013. PMID 23843690.
- Stapleton F, et al. The incidence of contact lens-related microbial keratitis in Australia. Ophthalmology. 2008. PMID 18538404.
- Carnt N, et al. Acanthamoeba Keratitis Risk Factors for Daily Wear Contact Lens Users: A Case-Control Study. Ophthalmology. 2023. PMID 35952937.
- Flores-Sánchez BC, et al. Acute angle closure glaucoma. Br J Hosp Med (Lond). 2019. PMID 31822188.
- Ah-Kee EY, et al. A review of drug-induced acute angle closure glaucoma for non-ophthalmologists. Qatar Med J. 2015. PMID 26535174.
Further reading
- Conjunctivitis: types, duration and treatment
- Foreign body in the eye: the right things to do
- Stye or chalazion: how to tell them apart
- Book an appointment at the Cachan practice
Consultations in Cachan, Paris 13 and at the Sorbonne Saint-Michel practice (Paris 5th) · Tel. +33 1 45 47 08 11
Have a persistent red eye examined
If your red eye persists, comes with pain, a drop in vision or if you wear contact lenses, an examination can identify the cause and rule out an emergency. For this, book an appointment with Dr Moïse Tourabaly. He sees patients at the Cachan and Paris 13 practices, and at the Sorbonne Saint-Michel ophthalmology practice (Paris 5th). In case of intense pain or a sudden loss of vision, you should see an ophthalmologist the same day or, failing that, go to an eye emergency department (see our page on eye emergencies).
This article is for informational and educational purposes. It does not replace a medical consultation. Any red eye that is painful or associated with a drop in vision should be assessed by an ophthalmologist. Sources: articles indexed on PubMed; recommendations of the French Society of Ophthalmology (SFO).
Written and reviewed by Dr Moïse Tourabaly, ophthalmic refractive surgeon — former chef de clinique (Quinze-Vingts National Eye Hospital).
Last updated: October 11, 2026
