Eye drops are designed for local action, but a significant amount of the medication does not stay on the eye’s surface. This excess drug can be absorbed into the body’s circulation and travel throughout the system. The potential for the drug to reach the untreated eye or cause systemic effects elsewhere in the body is real, primarily due to the body’s internal drainage and absorption mechanisms. Understanding this systemic pathway is essential for safely using eye drop medications.
The Systemic Pathway: How Drops Affect the Body
The eye has an efficient drainage system that clears away excess fluid and medication. When a drop is placed on the surface, a large portion drains almost immediately through the nasolacrimal duct. This small channel connects the corner of the eye directly to the nasal cavity.
Once the drug flows into the nasal cavity, it encounters a highly vascularized mucous membrane. This area is rich with blood vessels, allowing for rapid and direct absorption into the bloodstream. Up to 80% of the applied drug can be absorbed systematically through this drainage pathway.
This absorption route bypasses the liver, which is the body’s primary filter for breaking down drugs. Bypassing this “first-pass metabolism” allows a higher concentration of the active drug to reach the systemic circulation. The ocular surface, particularly the conjunctiva, also contains blood capillaries that absorb the medication, further contributing to systemic exposure.
Eye drops contain a high concentration of active ingredients, far more than the eye can absorb locally. This necessary overdose ensures a therapeutic amount penetrates the eye tissue. However, it results in a large excess that is available for systemic absorption and potential effects in distant organs.
Medications That Require Extra Caution
Certain classes of eye drops carry a higher risk of causing systemic side effects due to their potency. Glaucoma medications, particularly beta-blockers like timolol, are a primary example because they affect the cardiovascular and respiratory systems. When absorbed systemically, these drugs can slow the heart rate or lower blood pressure, which concerns patients with existing heart conditions.
Beta-blocker eye drops can also worsen respiratory issues in individuals with asthma or chronic obstructive pulmonary disease. Topical application of these drugs results in measurable plasma concentrations, demonstrating their systemic reach.
Corticosteroids
Corticosteroid eye drops, often used to treat inflammation, require careful consideration, especially with prolonged use. While topical use reduces the likelihood of systemic side effects compared to oral steroids, absorption can still occur. Systemic exposure can potentially lead to elevated blood sugar levels or, in pediatric patients, iatrogenic Cushing’s syndrome.
Cycloplegic Drops
Other medications, such as cycloplegic drops used to dilate the pupils, can also cause systemic effects. These effects include changes in heart rate, especially in children. This is often because their small body weight is not accounted for in standard ophthalmic dosing.
Essential Techniques to Minimize Cross-Eye Effects
Simple techniques can significantly reduce the amount of medication that enters the bloodstream. The most effective method is punctal occlusion, which physically blocks the primary drainage route. To perform this, immediately after placing the drop, gently press a finger against the bony area near the inner corner of the eye, next to the nose.
This gentle pressure should be held for at least two to three minutes, closing the opening of the nasolacrimal duct (the punctum). This action prevents the medication from draining into the nasal passages where it would be rapidly absorbed. This technique has been shown to reduce the systemic absorption of some drugs by over 65%.
Closing the eye for a few minutes immediately after instillation and avoiding blinking helps. Keeping the eye gently closed maximizes local absorption into the intended eye tissue. Use only a single drop, as the eye can only hold a small volume of liquid, and any excess will simply run off. Blotting any excess liquid that runs onto the cheek further reduces the chance of systemic absorption.