01.09.3 Eye

When a drug is a liquid/drop form, it is placed on the conjunctiva, and the released drug dissolves in the tears, and following blinking, is distributed to the site of absorption, the cornea. Local effects usually require absorption of the drug through the cornea. Although eye drops are used for their local effects, absorption can lead to systemic effects as the drug is drained from the eye through the nasolacrimal canal. As this drainage is not subject to first pass liver metabolism, it can lead to systemic side effects. Absorption may be increased by corneal infection or trauma, and dosage and care is needed in these conditions. An example of a drug that is used as eye drop is the β-blocker timolol, which is used in the treatment of glaucoma, high intraocular pressure. The highest concentration of timolol is in the eye, and that is where the predominant effects are. Major systemic effects are not observed after timolol is applied topically to the eye.

Drugs can also be administered to the eye in the form of an ointment. A small amount of ointment is put along the inside of the lower eyelid. Then, blinking will spread the ointment over the cornea, and the drug will be absorbed. This provides a more prolonged contact time with the cornea, and more prolonged absorption. With ointments, the application to the eye is less often than with drops.