Most eye drops are primarily water, but the active ingredients mixed into that water vary widely depending on what the drops are designed to do. A bottle of lubricating drops, allergy drops, and redness-relief drops each contain fundamentally different chemicals that work in different ways. Here’s what’s actually inside each type.
Lubricating Drops (Artificial Tears)
The active ingredients in artificial tears are water-soluble polymers that coat the surface of your eye and hold moisture in place. The most common are cellulose derivatives like carboxymethylcellulose and hydroxypropyl methylcellulose, typically at concentrations between 0.2% and 2.5%. These are essentially plant-based thickeners that mimic the slippery, protective quality of natural tears.
Other lubricating drops use liquid polyols instead. Glycerin, polyethylene glycol, and propylene glycol all appear at concentrations of 0.2% to 1%. Polyvinyl alcohol (0.1% to 4%) and povidone (0.1% to 2%) are also FDA-approved lubricant ingredients. Some formulas combine two polymers for a thicker, longer-lasting layer. The choice between them mostly affects how thick the drop feels in your eye and how long the relief lasts.
Allergy Drops
Allergy eye drops work by blocking histamine, the chemical your immune system releases during an allergic reaction. The most widely available over-the-counter option contains ketotifen, which does double duty: it blocks histamine receptors on the surface of your eye and also stabilizes mast cells, preventing them from releasing histamine in the first place.
Prescription allergy drops use similar strategies with different active ingredients. Olopatadine, one of the most commonly prescribed, is both a histamine blocker and a mast cell stabilizer. Others like epinastine and alcaftadine focus primarily on blocking histamine receptors, while drops containing ingredients like nedocromil or lodoxamide work purely as mast cell stabilizers, stopping the allergic reaction before it starts rather than blocking its effects after the fact. Your eye doctor picks between these based on how severe your symptoms are and how often they flare up.
Redness-Relief Drops
Redness-relief drops contain vasoconstrictors, chemicals that shrink the tiny blood vessels on the white of your eye. The most common active ingredients are tetrahydrozoline and naphazoline. These belong to a class of drugs that stimulate receptors on blood vessel walls, forcing the vessels to tighten. When the swollen vessels shrink, the pink or red color disappears.
The catch is that these drops can cause rebound redness. When the drug wears off, your blood vessels dilate again, sometimes more than before you used the drops. Over time, this cycle can lead to persistent redness that’s worse than what you started with. A newer prescription alternative uses a different active ingredient, brimonidine tartrate, which works through a similar mechanism but is formulated to reduce the rebound effect.
Prescription Drops for Eye Pressure
Glaucoma drops lower the pressure inside your eye, and they use several different classes of active ingredients to do it. Prostaglandin analogs like latanoprost, travoprost, and bimatoprost are typically the first choice. They work by increasing the drainage of fluid out of your eye. Beta-blockers such as timolol reduce the amount of fluid your eye produces in the first place. Carbonic anhydrase inhibitors like dorzolamide also slow fluid production through a different chemical pathway. Some people use combination drops that contain two of these ingredients in a single bottle.
Preservatives
Any multi-dose bottle of eye drops (one you use more than once) needs a preservative to prevent bacterial growth after you open it. The most common is benzalkonium chloride, or BAK, found in roughly 70% of all topical eye drops. BAK kills bacteria effectively, but it also has well-documented downsides: it damages cells on the surface of the eye and breaks down the lipid layer of your tear film, which can actually make dry eyes worse over time. Most eye doctors recommend using drops with BAK no more than four times a day.
Gentler alternatives exist. Polyquad (polyquaternium-1) is a much larger molecule, about 27 times the size of BAK, which makes it harder for it to penetrate and damage your eye’s surface cells. It’s commonly found in contact lens solutions and some artificial tear brands. Another option, sold under the name Purite, is a stabilized oxychloro complex that breaks down into sodium, chloride ions, oxygen, and water once it hits your eye. It kills microbes through oxidation but is less harmful to human cells because our cells have built-in antioxidant defenses that bacteria lack.
Preservative-free drops come in single-use vials. You twist one open, use it, and throw it away. These are the safest option if you need drops more than four times daily or have sensitive eyes.
Inactive Ingredients
Beyond the active ingredient and preservative, every eye drop contains a supporting cast of inactive ingredients. Buffers like boric acid and sodium citrate keep the pH close to natural tears (around 7.4) so the drop doesn’t sting. Sodium chloride adjusts the saltiness to match your body’s fluids, a property called tonicity. Purified water makes up the bulk of the solution. Some drops also include stabilizers that keep the active ingredient from breaking down before the expiration date. These inactive ingredients don’t treat anything directly, but without them the drops would be painful to use or ineffective.
Contamination Risks
Eye drops are meant to be sterile, and that matters more than with most over-the-counter products because liquid applied directly to the eye bypasses many of your body’s natural defenses. In late 2023, the FDA warned consumers to stop using 26 over-the-counter eye drop products after investigators found insanitary conditions and positive bacterial test results at the manufacturing facility in India. Several major store brands were included in the subsequent recalls. Contaminated eye drops can cause serious infections that lead to partial vision loss or blindness. Buying from established brands, checking for FDA recalls, and never using drops past their expiration date are the simplest ways to reduce that risk.