A red eye can be alarming but is commonly just a sign of conjunctivitis or another minor eye condition. If it is painful, there may be a more serious problem. In any case, you should see your GP for advice.

See your GP as soon as possible if you have a painful red eye or if you have other symptoms including reduced vision, sensitivity to light, a severe headache and feeling sick (nausea).

This page aims to give you a better idea of the cause of your red eye. However, it should not be used to self-diagnose your condition. Always see your GP for a proper diagnosis and treatment.

Common causes of a painless red eye

The most likely cause of a painless red eye is conjunctivitis. The next most likely cause is a burst blood vessel.

Conjunctivitis

Conjunctivitis is inflammation (swelling and irritation) of the conjunctiva, the thin layer of tissue covering the eyeball and inner surfaces of the eyelids. This causes the blood vessels on the eye to swell, making the eye look bloodshot and feel gritty.

Conjunctivitis can be caused by an irritant, such as chlorine or dust, an allergy (for example, to pollen), or an infection.

Allergic conjunctivitis will usually affect both eyes and is intensely itchy, while viral conjunctivitis tends to affect one eye first (which becomes watery), with redness developing in the second eye a few days later. Most cases of viral conjunctivitis occur along with a common cold. Bacterial conjunctivitis will usually cause a sticky discharge from the eye and crusting around the eyelids.

Treatment will depend on which type of conjunctivitis you have. In some cases, the condition may not require any treatment, but you may be prescribed some eyedrops. You can ease your symptoms at home by following some simple advice:

• avoid touching the eye and spreading any infection to the other eye
• hold a clean, cold damp face flannel to the eye to soothe and cleanse it
• do not wear make-up or contact lenses until the conjunctivitis has cleared
• do not share towels, flannels and pillow cases with others in the home while you have conjunctivitis

Burst blood vessel in the eye

Straining or coughing can sometimes cause a blood vessel to burst on the eye surface, causing a bright red blotch. This is called subconjunctival haemorrhage.

It can look alarming, especially if you are taking medication such as aspirin or warfarin (these reduce the blood’s ability to clot, which can exaggerate the redness), but should clear up on its own within a few weeks.

Common causes of a painful red eye

If your red eye is painful or you have other symptoms such as loss of vision, the cause is likely to be one of the following.

Iritis 

Iritis means inflammation of the iris (the coloured part of the eye). It is also known as anterior uveitis. Usually no cause is identified, although it can sometimes be due to an underlying condition or an infection.

As well as a red eye, you may notice that your eye is oversensitive to light. You may also have a headache and blurred vision.

Iritis usually responds quickly to treatment and rarely causes severe problems.

Acute glaucoma

Acute glaucoma is a serious condition where there is a sudden increase in pressure inside your eyeball. The eye will probably be severely painful and watering, and you may feel sick and see halos around lights. Your vision may be blurred or cloudy.

If your GP thinks you may have acute glaucoma, they will refer you to an eye specialist (ophthalmologist) immediately.

An ulcer on the cornea

A corneal ulcer is usually caused by a bacterial or viral infection and will feel like there’s a particle in your eye. Bacterial corneal ulcers are usually seen in people who wear contact lens. Viral corneal ulcers are often seen in people who frequently get cold sores. The eye may also be oversensitive to light.

Your GP will refer you to an eye specialist for treatment.

A scratch to the cornea or particle in the eye

If there is a particle in your eye, such as a piece of grit, your GP or a hospital doctor at the A&E department will try to remove it. They will first put anaesthetic eye drops into your eye to numb it and prevent any pain.

If the particle has scratched your eye, it may feel a bit uncomfortable when the anaesthetic eye drops have worn off. You may be given antibiotic eye drops or ointment (such as chloramphenicol) to use for a few (typically five) days to reduce the risk of infection while it heals.

See eye injuries for more information.