Eye drops are a frequently used pharmaceutical format intended to treat various conditions affecting the front part of the eye. This method of drug delivery is specifically designed to concentrate medication on the eye’s surface, minimizing exposure to the rest of the body. Understanding how this administration route is classified and how the medication interacts with the eye’s anatomy provides clarity on its effectiveness and proper use.
The Classification of Ocular Delivery
The delivery of medication via eye drops falls under the Topical route of administration, which involves applying a substance directly to a body surface. In this specific context, it is more accurately termed the Ocular or ophthalmic route, differentiating it from topical application to the skin or mucous membranes elsewhere. This classification means the medication is intended to act primarily at the site of application: the external surface of the eye.
This local approach contrasts sharply with other routes, such as the oral (enteral) route, where a drug is swallowed and absorbed through the digestive tract. It also differs from parenteral routes, like injections, which deliver medication directly into the bloodstream or tissues. The advantage of topical ocular delivery is achieving a high concentration of the drug on the eye’s surface immediately, which is ideal for treating conditions like conjunctivitis or dry eye.
The goal is to limit the drug’s overall presence in the body, focusing the therapeutic effect on the anterior segment of the eye. Because the eye is a relatively isolated structure, a local delivery method is preferred to achieve maximum localized efficacy with fewer systemic side effects. This method is the most common way to administer ophthalmic medications, accounting for approximately 90% of all eye drug delivery.
How Eye Drops Are Absorbed Locally
Once an eye drop is administered, the drug must penetrate the eye’s protective layers to reach its target tissues, beginning with the tear film. The tear fluid volume is small (approximately 7 microliters), meaning most of the drop is rapidly lost due to spillover, blinking, and tear drainage. Consequently, only a fraction of the original dose is available for absorption into the eye structures.
The medication primarily enters the eye through two main pathways: the cornea and the conjunctiva. The cornea is the transparent, dome-shaped front surface of the eye and is the main route for drugs targeting internal eye structures. Its multi-layered structure, particularly the outer, fatty (lipoidal) epithelium, acts as a significant barrier, limiting the passage of many therapeutic agents.
The second pathway is the conjunctiva, the thin, vascular membrane covering the white of the eye and the inner surface of the eyelids. Although the conjunctival surface area is approximately 17 times larger than the cornea, it is considered a less productive route for deep ocular delivery. While drugs cross the conjunctiva easily, its rich network of blood vessels quickly carries the medication away into the general circulation. This loss into the bloodstream means that drug bioavailability inside the eye is often less than 5% of the administered dose, reducing the amount available to treat the eye condition itself.
Preventing the Drug from Entering the Body
Despite the intention for a local effect, the tear drainage system provides a direct pathway for the medication to enter the systemic circulation. The lacrimal drainage system begins with small openings, called puncta, located at the inner corner of the upper and lower eyelids. These puncta lead to tiny canals that drain into the nose and throat, which is why patients can sometimes taste eye drops shortly after administration.
When the medication drains through this system, it is absorbed by the vascular tissues of the nasal and pharyngeal mucosa, introducing the drug into the bloodstream. This unintended systemic absorption can lead to side effects in other parts of the body, particularly with certain glaucoma medications like beta-blockers. Systemic absorption also reduces the concentration of the drug available to treat the eye, making the treatment less effective.
To minimize this loss and prevent unwanted systemic effects, patients can perform punctual occlusion immediately after administering the drop. This involves gently closing the eye and placing a clean index finger over the inner corner of the eye, directly over the tear duct opening. Applying light pressure for 60 seconds to two minutes physically blocks the drainage route. This action keeps the medication on the surface of the eye for a longer period, maximizing local absorption and therapeutic benefit while reducing the amount that enters the body.