Why is eye red on one side?

A red eye on just one side is pretty common and usually not a major alarm but it's important to assess the cause. The redness is due to dilated or inflamed blood vessels on the white of the eye (sclera). Common causes include: 1) Conjunctivitis (Pink Eye): Can start in one eye before spreading. Itchy watery with a gritty feeling. Can be viral bacterial or allergic. 2) Subconjunctival Hemorrhage: A bright red patch caused by a broken blood vessel. Looks scary but is usually harmless caused by coughing straining or even rubbing the eye. 3) Foreign Body or Injury: Something in the eye or scratching it. 4) Blepharitis: Inflammation of the eyelid margin. 5) Uveitis or Keratitis: More serious inflammation inside the eye or cornea often with pain and light sensitivity.

If it's just redness with no pain itchiness or vision changes it might resolve on its own. Use artificial tears avoid rubbing and get rest. But if you have severe pain vision changes sensitivity to light headache or nausea see a doctor or ophthalmologist immediately. It could signal issues like acute glaucoma or a corneal ulcer. Don't use old prescription eye drops. The American Academy of Ophthalmology's red eye guide is a trustworthy resource.

I've had this happen from staring at a screen too long on one side (weird I know) and from a tiny eyelash that got stuck. First wash your hands and gently examine your eye in a mirror under good light. Look for an eyelash or speck. You can try rinsing it with sterile saline solution.

If it's itchy it's likely allergies. If it's painful or feels like sand is in it it could be an infection or scratch. A subconjunctival hemorrhage (the blood spot) is totally harmless but takes 1-2 weeks to clear—it just looks bad. My rule of thumb: If it's not better in 24-48 hours with rest and lubrication or if any other symptoms appear get it checked out. Never share towels or pillows when one eye is red to prevent spreading potential infection. And for goodness sake take out your contact lenses if you wear them!