Reviewed by Dr. Davinder Sidhu, OD
[Image: preserved drugstore bottle next to a preservative-free multidose bottle on the shelf, phone photo fine]If you use eye drops most of the week, preserved drops are probably the wrong bottle for you. Because what the preservative does to the surface of your eye, several times a day, every day.
The preservative in most multidose bottles is benzalkonium chloride, BAK on the label. It's on the shelf for a real reason. It stops the bottle growing bacteria once you've opened it. However that same chemical keeps working after the drop leaves the bottle and lands on your cornea.
What BAK is
BAK is a detergent. It's in the same broad chemical family as a lot of surface cleaners. Detergents break down fatty membranes, which is how they kill bacteria, the bug's outer membrane comes apart and it dies. It's cheap and it's stable, and has been the standard preservative for decades.
The cells on the front of your eye are also wrapped in a fatty membrane, the preservative doesn't distinguish between a bacterium in the bottle and one of your own surface cells.
What it does to the cornea
When the preserved drop hits the eye, the BAK acts on the outermost layer of cells. Studies have found the same thing repeatedly: BAK exposure increases cell death at the surface, slows how quickly the surface heals, and actually makes the barrier more permeable than it should be.
One drop on a healthy eye isn't a problem. The surface cells turn over constantly, which is the point people make when they say preserved drops are approved and safe. For someone who uses drops now and then, there's no real issue. The concern is repeated exposure over months and years.
The goblet cells are the part that matters most
Goblet cells sit across the surface of the eye and the inner lining of the lids, producing mucin, the mucus layer that forms the base of the tear film. Without enough mucin, your tears won't spread evenly across the eye.
BAK reduces goblet cell numbers with repeated exposure. That's a slower problem than a scuffed surface cell, which gets replaced in a day or two, because the goblet cell recovery takes longer as long-term exposure can lower its baseline.
Less mucin means the tear film breaks up faster between each blink, which means more dry spots and more reaching for the bottle.
Inflammation also adds to it
Damaged surface cells with an unstable tear film will result in inflammation. The surface releases inflammatory signals, and more reactive tissue tends to respond harder to the next irritant. So the next preserved drop goes onto a surface that's already inflamed rather than a clean starting point. This is one of the reasons why dry eye tends to build on itself over time.
Why a healthy eye copes and a dry one doesn't
This is why it is important to separate "safe for most people" from "safe for you."
A healthy eye has good tear volume and good turnover and a preserved drop gets diluted and flushed off the surface fairly quickly, so the BAK doesn't sit there long or at high concentration.
A dry eye has lower tear volume and slower turnover. The same thing that makes the eye dry, (not enough tears) also means the preservative stays on the surface longer and at a higher effective concentration. So those who most need drops are also the same people whose eye is least able to clear the preservative in them.
Someone using a preserved drop occasionally gets a few low exposures a year. Someone dosing four to eight times a day is delivering BAK to an already-struggling surface many times a week. It's the same preservative but a very different exposure, and daily users are the ones getting the most of it.
What to do about it
If you use drops rarely, a preserved bottle is fine and it costs less, so there's no need to change.
If you're a daily user, or getting there, it's worth switching to preservative-free drops. They come as single-use vials or as multidose bottles with a one-way valve or filter that keeps the contents sterile without a chemical preservative. You get the sterility without putting a detergent on your cornea every time. The surface cells get a chance to recover and the tear film can do its job.
If you want the full picture on making the switch, our complete guide to preservative-free eye drops walks through where to start.
Regular and preservative-free drops are usually sold next to each other as if they're the same product at two price points. For occasional use, that's good enough. For daily use, they aren't really the same thing.
About the Reviewer
Dr. Davinder Sidhu is an optometrist based in British Columbia with a focus on dry eye management and preservative-free solutions. Learn more at TheGenuwineOD.com or follow him on Instagram and Facebook.
```