Which preservative is listed as an alternative to BAK to reduce ocular surface toxicity?

• A. Sodium chloride
• B. Chlorobutanol
• C. Purite
• D. Polyquad (polyquaternium-1)

Answer: (C)

Preservatives in eye drops are chosen to kill microbes while minimizing damage to the ocular surface, especially with long-term use. Benzalkonium chloride (BAK) is effective but can cause tear film instability, goblet cell loss, and surface irritation if used chronically. Purite stands out as an alternative because it’s photolabile: when exposed to light and the tear film, it degrades into harmless components—saline, water, and oxygen—so the active preservative isn’t left behind on the eye after dosing. This means you get antimicrobial protection in the bottle, but much less persistent exposure to the ocular surface, reducing toxicity risk during chronic therapy.

Sodium chloride isn’t a preservative at all, it’s simply saline. Chlorobutanol is an older preservative with a higher potential for irritation and toxicity. Polyquad is another gentler option than BAK, but it remains in the tear film rather than breaking down like Purite, so Purite’s breakdown into benign substances makes it a clearer choice for reducing ocular surface toxicity.