Allergies or eye infection: how to tell the difference?

26 February 2026

Eye redness is one of the most common complaints in ophthalmology consultations. Many people assume it is “conjunctivitis” without realizing that there are different causes behind a red eye.

Among the most common are ocular allergies and infections. Although they may present similar symptoms, their origin and treatment are different, so an accurate diagnosis is essential.

Ocular allergy: an immune system reaction

Ocular allergy occurs when the eye overreacts to external substances such as dust, pollen, mites, pet dander, or environmental changes. This reaction causes inflammation of the conjunctiva, the membrane that covers the white part of the eye.

The most characteristic symptom of allergy is intense itching. It usually affects both eyes at the same time and is accompanied by clear tearing, mild eyelid swelling, and often nasal symptoms such as sneezing or congestion.

An important point is that ocular allergy is not contagious.

Rubbing the eyes constantly can worsen the inflammation and prolong the discomfort.

Ocular infection: caused by microorganisms

Eye infections can be caused by bacteria or viruses. In these cases, redness is usually accompanied by thick, yellowish or greenish discharge, and it is common for the eyelids to be stuck together upon waking.

Unlike allergies, an infection may start in one eye and then spread to the other. In addition, some infections—especially viral ones—are contagious and can be transmitted through direct contact.

A foreign body sensation, increased discomfort, and abundant discharge usually suggest an infectious cause rather than an allergic one.

Why is it important to differentiate them?

Although both conditions may seem mild, the treatment is different. Allergies are typically managed with anti-allergy eye drops and measures to avoid the trigger. Bacterial infections, on the other hand, may require specific antibiotics.

Self-medicating can delay improvement and even worsen the condition. The improper use of certain eye drops can mask symptoms or aggravate inflammation.

Warning signs

It is important to see an ophthalmologist if there is severe pain, marked light sensitivity, decreased vision, or if symptoms persist for more than two or three days without improvement.

A comprehensive eye examination allows the exact cause to be identified and the appropriate treatment to be established from the beginning.