A red eye is a very common condition, an eye that appears red due to illness or injury. Fortunately, many a time, the red eye is of short duration, not very troublesome, and resolves by itself.

Simple red eyes usually clear by remedies such as washing with plain cold water, applying cold packs to closed eyes, taking a short period of rest, etc.

Commonest cause is conjunctivitis, either due to allergy, or due to infection. Allergies can cause itchy, red eyes that appear swollen. Infection, on the other hand, can cause stickiness of the eye lids or eye lashes with some discharge in addition. While the very mild may clear by themselves, others will need specialist treatment.

Other causes of red eye, which need proper treatment are, keratitis (injury/infection of cornea), scleritis & episcleritis (involvement of white layers of eye).

Glaucoma or iritis can also cause redness of the eye, and these conditions would need immediate treatment. Acute glaucoma, if treatment is delayed, can lead to permanent loss of vision.

To prevent delay in treatment, any one of the following feature indicates that an urgent consultation with an ophthalmologist is important:

Along with redness of eyes, if there is: 

(1) pain in or around the eyes.
(2) watering from the eyes.
(3) photophobia, or inability/hypersensitivity to tolerate light.
(4) diminution of ability to see clearly, which was good until the onset of red eye.

Lubricating eye drops and general soothing eye drops usually give some relief in almost all the conditions. That could be a reason sometimes when we may delay seeking specialist consultation.
One big danger is in using steroid eye drops, while very essential in treating some of the conditions mentioned above, has exactly the opposite and very harmful result in some other conditions. Hence, only a well trained ophthalmologist can identify these conditions quickly and advise accordingly.