There are many causes for red eyes. Some are less serious than others.
Quite a few associated symptoms need to be taken into account when looking to diagnose why someone has red eyes. If one's vision is decreased or there is sensitivity to light or a discharge coming out the eye, then an ophthalmologist needs to be consulted as soon as possible. There could be something serious such as uveitis or infective (viral or bacterial ) conjunctivitis present, or even an ulcer on the cornea and all these conditions need urgent medical treatment.
Sometimes red eyes are caused by blepharitis (infection of the eyelid) or allergies and these conditions are less serious but still annoying.
The symptoms of Blepharitis can be treated by beginning lid hygiene techniques using a special eyelid cleanser and sometimes requires antibiotic ointments or even tablets over and above this. Allergies can be due to many different causes such as pollen, dust and chemical products. Allergies and dry eye can also be associated with red eyes which makes all the symptoms worse and requires lubricant eye drops to be added to the cleaning regimen and antibiotics.