Ocular allergy covers a series of immune-allergic inflammatory diseases of the ocular surface, with different degrees of involvement and severity. These pathologies are caused by a variety of IgE- and non–IgE-mediated immune mechanisms and may involve all parts of the external eye, including the conjunctiva, cornea, eyelids, tear film, and commensal flora. The most frequent is allergic conjunctivitis, a condition with different clinical forms that are classified according to the degree of involvement and the presence or absence of proliferative changes in the palpebral conjunctiva, associated atopic dermatitis, and mechanical stimuli by foreign bodies, including contact lenses. Treatment guidelines for allergic conjunctivitis propose a stepwise approach that includes medications for both ophthalmic and oral administration depending on symptom severity, allergic comorbidities, and degree of control. In the case of antihistamines, eye drops are the most prescribed ophthalmic formulations.
To avoid disrupting the delicate balance of the ocular surface, topical ophthalmic medications must be well tolerated. The primary aim of this article is to review the physicochemical characteristics and other features of excipients (preservative agents, buffers, pH adjusters, viscosity enhancers, wetting agents or cosolvents, antioxidants, tonicity adjusters, and osmo-protectants) and active compounds (ocular antihistamines) that must be considered when developing formulations for ophthalmic administration of antihistamines. We also provide a brief overview of antihistamine eye drops that could be of interest to professionals treating ocular allergy and encourage the use of preservative-free formulations when possible.

Key words: Antihistamine eye drops, Ocular allergy, Allergic conjunctivitis, Excipients, Preservatives, Ophthalmic drug administration, Ocular surface, Hyaluronate