This stands for benzalkonium chloride. BAK is a preservative found in many eye drops and rewetting drops. This preservative is necessary to keep bacteria from colonizing the bottle after being opened. Unfortunately, the preservative itself is a little harsh on the cornea. This is one of the reasons why we don’t recommend using drops more than four times a day. This is particularly important for our dry eye and glaucoma patients who may be taking numerous eye medications.  Fortunately, there are now preservative-free rewetting drops available. Many of the glaucoma medications are now available in more expensive preservative-free versions (Zioptan and preservative-free Cosopt).

Dr. Timothy Root is a practicing ophthalmologist and cataract surgeon in Daytona Beach, Florida. His books, video lectures, and training resources can be found at:


  1. BAK is a preservative common in many medicated eye drops. Some people seem to be sensitive to the stuff. This is a problem if you are taking multiple drops (as many glaucoma patients do) or have preexisting surface issues (like dry eye).

    Oddly enough, BAK isn’t always bad. By irritating the corneal surface, this additive may actually improve penetration of certain medicines inside the eye.


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