Drug bioavailability from topically applied ocular drops. Does drop size matter?

The application of drops containing ocular medicines to the conjunctival sac is the most common method of drug delivery to the anterior segment of the eye. Although this route of application seemingly displays numerous advantages, obtaining effective drug concentration at its site of action is challenging. The bioavailability of a topically applied drug depends on various factors related to the eye, to the drug and formulation, to the drop, and to the patient. The present article discusses their relative significance. From a drop applied to an eye, at most 5% of a drug dose enters the ocular structures. Of utmost importance for effective ocular drug delivery are patient compliance and the physicochemical properties of the drug. For a given concentration of an active substance, drop size may determine drug adverse effects but does not influence its efficacy.