What should you know about only one eye is red?

A small bright-red patch on the white of the eye can follow minor irritation or straining and may not affect sight. Conjunctivitis can cause redness, watering, discharge, gritty discomfort and eyelids sticking together; it may begin in one eye and spread. Allergic conjunctivitis more often causes itching and watering, usually with other allergy symptoms. A scratch, foreign body, contact-lens problem or inflammation inside the eye can be more serious. The appearance alone cannot identify the cause.

Do not rub the eye. Wash your hands, avoid sharing towels and pause contact lenses until symptoms have been assessed or fully settled. If dust or a loose irritant is present, rinse gently with clean water; do not try to remove something stuck in the eye. Avoid using leftover antibiotic or steroid drops. A clinician can examine the surface, check vision and choose treatment suited to the cause.

Seek urgent eye care for moderate or severe pain, reduced or blurred vision, marked light sensitivity, a chemical splash, an eye injury, a foreign body that will not wash out, or redness with nausea and headache. Contact a clinician promptly if you wear contact lenses, develop increasing discharge or swelling, or symptoms are worsening.

Could this be conjunctivitis, allergy, irritation or an injury? Should I stop contact lenses, and which drops are appropriate? What change in pain, discharge or vision should prompt urgent review?