Why Should LHON Patients Avoid BAK-Preserved Eye Drops?

Benzalkonium chloride (BAK) is a very common preservative used in both over-the-counter and prescription eye drops to prevent bacterial growth. While BAK is generally safe for the public, some researchers recommend that people with Leber Hereditary Optic Neuropathy (LHON) avoid it as a precaution ^[1].

This recommendation is based on laboratory studies showing that BAK can be toxic to mitochondria—the energy-producing “batteries” of our cells ^[1]. Because LHON is a disease driven by preexisting mitochondrial problems, exposing already vulnerable cells to further stress is considered a theoretical risk ^[1].

It is important to know that this risk is theoretical. There is no clinical evidence proving that using BAK eye drops causes or worsens vision loss in people with LHON ^[1]. The advice to avoid BAK is a protective measure based on laboratory experiments, not on real-world vision outcomes.

The Science: BAK and Mitochondria

In patients with LHON, genetic mutations affect a specific part of the mitochondrial energy machinery known as Complex I ^[2]. Because of this defect, the retinal ganglion cells (the nerve cells in the eye that carry visual information to the brain) struggle to produce enough energy and are highly vulnerable to chemical damage known as oxidative stress ^[3].

In laboratory settings, researchers have found that BAK directly inhibits this exact same machinery—Complex I ^[1]. When isolated cells are exposed to BAK in the lab, they experience a drop in energy production and an increase in oxidative stress ^[1][4].

Crucially, one laboratory study compared normal cells to cells carrying LHON genetic mutations and found that the LHON cells were more sensitive to the toxic effects of BAK ^[1]. Because LHON cells have less energy reserve to begin with, they are more easily overwhelmed by additional mitochondrial stressors ^[1][3].

Theoretical Risk vs. Real-World Reality

While the laboratory findings are clear, they do not perfectly translate to the human eye. In the lab, cells are directly soaked in BAK. In the real world, an eye drop is quickly diluted by tears and blinked away. It is not scientifically known whether enough BAK from an eye drop actually penetrates deep enough into the eye to reach the retinal ganglion cells or the optic nerve ^[1].

However, LHON is a complex disease where some people with the genetic mutation lose vision and others never do. Researchers believe environmental triggers—sometimes called a “second hit”—can push vulnerable cells past their breaking point ^[5]. Because the authors of the laboratory studies view BAK as a potential “second hit,” they advise that people with mitochondrial deficiencies minimize their exposure whenever possible ^[1].

Independent of LHON, it is well-established that frequent use of BAK can cause ocular surface irritation and dry eye symptoms ^[1]. For this reason alone, many eye doctors prefer preservative-free drops for patients who use drops regularly.

Where is BAK Found?

BAK is frequently found in multi-dose bottles of:

• Over-the-counter artificial tears and lubricating drops
• Redness-relief drops
• Allergy eye drops
• Prescription drops, particularly for glaucoma

Actionable Steps for LHON Patients

• Read the Labels: Always check the “inactive ingredients” list on any eye drop for benzalkonium chloride (sometimes abbreviated as BAC or BAK).
• Choose Preservative-Free (PF): Look for drops clearly labeled “Preservative-Free.” Note that “BAK-free” is not the same as preservative-free, as BAK-free drops may just use a different preservative.
• Follow Packaging Instructions: Preservative-free drops often come in small, single-use plastic vials. Because they lack preservatives, these vials can grow bacteria once opened and must be discarded after a single use according to the package directions.
• Consult Your Pharmacist: When buying over-the-counter drops, ask your pharmacist to help you identify a truly preservative-free option.
• Do Not Stop Prescriptions Abruptly: If you are currently taking prescription eye drops (like glaucoma medication) that contain BAK, do not stop taking them without speaking to your doctor first ^[1]. Stopping prescribed eye drops can be dangerous and cause immediate harm to your vision. Contact your ophthalmologist to ask if a preservative-free alternative is appropriate for you.