Artificial tears are not all the same. Their clinical value depends largely on the type of active ingredients they contain. Broadly, the main functional classes used in ocular lubricants include viscosifying/lubricating agents, osmoprotective and cytoprotective molecules, and lipid-based components.
Choosing the right formulation
Hyaluronic acid (sodium hyaluronate) is one of the most widely used ingredients in Europe. It is a highly effective humectant with strong water-retaining capacity, prolonged ocular surface residence, and favorable viscoelastic behavior. Its mucoadhesive properties improve surface wettability, tear film stability, and patient comfort, while some formulations may also support epithelial healing.
Cellulose derivatives such as carboxymethylcellulose (CMC) and hypromellose (HPMC) remain major reference compounds in artificial tears. These agents increase viscosity, improve retention time, reduce friction, and enhance corneal wettability. They are mainly used to provide durable aqueous lubrication, particularly in mild to moderate dry eye disease.
In practical terms, lubricating polymers mainly improve viscosity, retention, and comfort; osmoprotective agents help protect the ocular surface from hyperosmolar and inflammatory stress; and lipid-based formulations are especially useful when evaporation and meibomian dysfunction are key drivers of disease.
Other classical lubricants, including polyvinyl alcohol (PVA), povidone, polyethylene glycol (PEG), and propylene glycol, primarily act as demulcents and surface protectants. They reduce irritation and mechanical friction while helping preserve a more stable tear film. Hydroxypropyl guar (HP-guar) adds a more advanced technological effect by forming a soft gel matrix in situ, thereby prolonging ocular surface contact and improving retention of co-formulated agents. Carbomer, because of its higher viscosity and longer residence time, is particularly useful when prolonged protection is required, especially in more symptomatic patients or for nighttime use.
Osmoprotection
A second major group includes osmoprotective and cytoprotective agents, which are especially relevant in dry eye disease associated with tear hyperosmolarity and epithelial stress. Trehalose is the best-known example, with documented osmoprotective, anti-apoptotic, and surface-protective effects. Other osmoprotectants such as L-carnitine, erythritol, betaine, sorbitol, and in some formulations glycerin, help protect corneal and conjunctival epithelial cells from hyperosmotic damage. Their main contribution is biological rather than rheological: they do not simply thicken the drop, but help preserve cellular homeostasis.
Finally, lipid-containing eye drops target the superficial lipid layer of the tear film. Ingredients such as mineral oil, castor oil, phospholipids, and triglycerides are designed to reduce evaporation and improve tear film stability. These formulations are particularly relevant in evaporative dry eye and meibomian gland dysfunction (MGD), where aqueous supplementation alone is often insufficient.