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).