What can cause a red eye?
A red eye can result from dryness, allergy, infection, foreign body or more serious intraocular problems. Possible causes, safe initial assessment, diagnosis and emergency warning signs are explained in depth.
Possible causes, assessment and diagnosis
1. Meaning of the symptom or result A red eye can result from dryness, allergy, infection, foreign body or more serious intraocular problems. Severe pain, light sensitivity, reduced vision, trauma, chemical exposure or marked symptoms in a contact-lens wearer need urgent care. Ear, nose, eye, mouth, skin and nerve complaints can look similar despite different causes. Self-started antibiotics, steroids, eye or ear drops and herbal products may delay diagnosis or cause harm. 2. Information that helps distinguish causes Record exact onset, sudden or gradual development, constant or intermittent pattern and change in severity. Note fever, weight change, rash, bleeding, breathing, urine, stool, sleep and appetite. Tell clinicians about prescribed and non-prescribed medicines, supplements, allergies, possible pregnancy, recent infection, travel, trauma and family history. For measurements, record the device, method, time and repeat result; one home reading is not definitive. A photograph or diary may help but cannot replace examination and appropriate testing. 3. How diagnosis progresses A clinician first excludes urgent risks and then narrows possibilities through history and examination. Not everyone needs the same blood test, imaging or antibiotic. Benefit is balanced against false positives, radiation, invasiveness, cost and whether the result will change management. Persistent symptoms may need reassessment despite a normal test, and an abnormal value alone is not a disease name. Sometimes the course over time and response to care clarify the diagnosis.
Safe care, emergency warnings and mistakes
1. Safe initial care Without red flags, rest, adequate fluid, regular light meals and avoiding known aggravators may support many short-lived complaints. Fluid and salt advice must be individualized in heart, kidney or liver disease. Do not use another person's medicine, leftover antibiotics, adult medicines in children, megadose vitamins or overlapping painkillers. Keep a symptom record and seek care if symptoms persist, recur or worsen. 2. When emergency help is needed Severe breathing difficulty, chest pressure, new paralysis or speech trouble, fainting, seizure, confusion, uncontrolled bleeding, bloody vomit, black stool, sudden vision loss, very severe or rapidly worsening pain and severe allergy signs are emergencies. Infants, pregnancy, immune suppression, older age and serious chronic disease lower the threshold for assessment. Contact local emergency services if safe transport is not possible. 3. Common mistakes Choosing the most frightening search result, diagnosing from one sign, reading a test without its range and stopping prescribed treatment at random are unsafe. Natural does not mean harmless, particularly in pregnancy, childhood or with medicines. A social-media experience is not treatment evidence when age, disease and dose differ. Separate screening from diagnosis, risk factor from disease and temporary variation from persistent disorder. 4. Limits of this information This article provides general education, not individualized diagnosis, dosage or treatment. Reliable decisions combine examination, current guidance, personal preferences and specialist input where needed. Contact primary care or the relevant professional when symptoms are unusual, impair daily function, last longer than expected or cause concern.