eye drop

An eye drop is the most common eye medicine: a sterile liquid you instill one drop at a time into the pocket between the lower eyelid and the eyeball. It seems trivially simple, but a single drop is actually a tightly engineered sterile solution that has to be safe to place directly on a living, sensitive surface.

A typical drop is an aqueous solution (sometimes an oily solution or a fine suspension) containing the drug plus excipients that make it eye-friendly: a buffer to hold a comfortable pH, a tonicity agent so it is roughly isotonic with tears (to avoid stinging), often a viscosity-increasing polymer to help it linger, and — in multi-dose bottles — a preservative to keep it sterile between uses. The whole product must be free of particles and microorganisms because the eye has little defence against contamination.

A drop from a standard bottle is about 30–50 µL, but the eye can only hold roughly 7–10 µL of extra fluid; the rest spills onto the cheek or drains into the nose. This is why instilling two drops at once mostly wastes the second one, and why waiting several minutes between two different drops lets each be absorbed before the next washes it away. Most of any drug not absorbed locally drains into the nose, where it can be absorbed systemically — a route of unwanted side effects for some eye medicines.