Eye drops are a common, localized method of delivering medication directly to the eye’s surface. Although intended only for the treated eye, the body’s anatomy creates pathways that allow the drug to travel beyond the target area, potentially affecting the untreated eye or the entire body. This cross-effect occurs through two main mechanisms: the body’s internal circulatory system and the external, physical transfer of substances. Understanding these pathways helps ensure therapeutic benefit while minimizing unintended effects.
The Systemic Route of Absorption
The primary mechanism by which a drug applied to one eye can affect the other, or the body systemically, is through absorption into the bloodstream. When an eye drop is placed on the surface, a significant portion drains away rather than remaining in the eye. The natural tear drainage system, known as the lacrimal apparatus, quickly collects this excess fluid and medication.
Drainage occurs through small openings called the lacrimal puncta, located in the inner corner of the upper and lower eyelids. The fluid travels down the nasolacrimal duct into the nasal cavity or the back of the throat. This pathway is highly vascularized, allowing the drug to be readily absorbed by the mucous membranes and enter the systemic circulation.
Once the medication is in the bloodstream, it circulates throughout the body, bypassing the liver’s initial filtering process, which can lead to a higher concentration of the active drug. The circulating blood carries the medication to all organs, including the untreated eye. This can result in therapeutic effects or, more often, unintended side effects in the second eye or elsewhere in the body.
Systemic absorption is a major concern for certain drug classes. For instance, beta-blocker eye drops, commonly prescribed for glaucoma, can cause a drop in heart rate (bradycardia) or breathing difficulties (bronchoconstriction) if absorbed systemically. Strong steroid drops can also lead to systemic complications. It is estimated that up to 80% of an applied drug may diffuse into the systemic circulation via this nasopharyngeal route if not properly managed.
Minimizing Unintended Effects
Patients can significantly reduce the systemic absorption of eye drops through a technique called punctual occlusion. This technique physically blocks the medication’s drainage route into the nasal passage. Preventing the drug from reaching the highly absorptive nasal mucosa ensures more medication remains on the eye’s surface, maximizing the local effect.
To perform punctual occlusion, the patient must gently close the eye immediately after the drop has been applied. They should then use a clean finger to apply light pressure to the inner corner of the closed eye, near the bridge of the nose, directly over the lacrimal puncta. This physical pressure temporarily seals the drainage opening.
The pressure should be maintained for a specified period, typically ranging from one to three minutes, to ensure the medicine is fully absorbed into the eye tissues. It is also important to avoid blinking forcefully after instillation, as blinking can act like a pump, actively pushing the medicine into the drainage system. Combining gentle eyelid closure with punctual occlusion is the most effective way to keep the drug localized.
Addressing Physical Transfer and Contamination
Beyond pharmacological effects, one eye can affect the other through the physical transfer of substances, such as infectious agents. This is particularly relevant when an infection, like bacterial conjunctivitis, is present in one eye. The primary rule to prevent this is ensuring the dropper tip of the bottle never touches the eye, eyelid, or any other surface.
If the dropper tip makes contact with an infected eye, the bottle’s contents, or at least the tip’s exterior, can become contaminated with bacteria or viruses. Using that same bottle in the uninfected eye can then physically transfer the microbes, leading to a new infection. Studies have shown that microbial contamination on the dropper tip is a leading cause of potentially avoidable eye injury.
Specific precautions are necessary for patients treating an active infection or those using long-term medication who develop an infection in one eye. If one eye is infected and the other is not, the healthcare provider may recommend obtaining two separate bottles for routine medication to eliminate cross-contamination risk.
Consistent handwashing before and after drop application is essential. Additionally, immediately discard any bottle where the tip has accidentally touched the eye to prevent physical transfer.