Many things. Eye drops are either medicated or not. All preserved drops contain a chemical that keeps it from becoming contaminated. The remainder of the chemicals are determined by the specific intent of the manufacturers to treat the particular condition. You should never attempt to create your own eye drops.
Eye Drops. "eye drops" is a term that applies to many different formulations depending on the treatment purpose. The drops contain the active ingredient, say for gaucoma, or antibiotic, or steroid, or antihistamine and added to this is a preservative. Moisturizers/lubricants contain methylcellulose derivatives. Eye drops are safe when used properly according to the advice of an experienced physician.