Allergic conjunctivitis occurs in people who are sensitive to certain allergens, such as grass pollen or animal fur. The allergens dissolve in the tear film and cause an allergic reaction in the conjunctiva (involving the release of histamine from mast cells). Usually, allergic conjunctivitis is seasonal and short-lived (e.g. it occurs only when pollen is in the air), but it can be longer-lasting if the person is allergic to something that is always around (e.g. house dust mites, animal dander).

Typically, a person will have itchy, red, watery eyes with lid swelling, and perhaps a runny or stuffy nose. Allergic conjunctivitis usually affects both eyes. It will usually improve without treatment, but a variety of drugs can be been used to relieve symptoms.

These include:

• Antihistamine eye drops or tablets/liquid – useful for rapid relief of symptoms. Tablets or liquid may be more appropriate if the person also has nasal symptoms • Sodium cromoglicate eye drops – prevent the release of histamine from mast cells. Sodium cromoglicate does not reduce symptoms of allergy that are already present, but can be used to prevent symptoms in someone who knows to expect them (i.e. ahead of the hayfever season).
• Antihistamine eye drops, tablets or liquid might be useful for symptom control in the first few weeks while waiting for the sodium cromoglicate to take effect, and for breakthrough symptoms. There is no evidence that any one treatment is better than another.