If worn longer than recommended or in people whose eyes are particularly sensitive, lenses may develop deposits on the surface of the lenses and cause irritation.

A major cause of blinding eye infections is spitting on a contact lens or putting the lens in one's mouth when no solution is available. This happens when a contact lens wearer must remove a lens or if a lens falls out of the eye and no solution is available. The best prevention is to have a small bottle of rewetting solution with you at all times. Never, never put your contact lenses in your mouth to lubricate them.

Some people use homemade or non-contact-lens saline solutions in an effort to save money. These solutions may cause severe irritation or blinding infections (some saline solutions are not sterile) and should never be used. Always use the correct contact-lens solution recommended by the fitter. If you want to change solutions, you should first check with your fitter because some solutions may be incompatible with certain lenses

It is not unusual to have some general irritation, even some redness, upon beginning contact lens wear. However, persistent redness, light sensitivity, pain, and blurred vision are the main signs and symptoms of potential eye problems. Depending on the cause, symptoms vary in intensity. In more serious infections, the pupil in the infected (red) eye may be smaller than the other pupil.

With a torn or broken lens or if there is something in the eye, there is usually a slight feeling of general irritation as if something is in the eye. There may be some associated redness.

With a poor-fitting lens or a lens that is old, there may be a slight irritation and redness associated with some blurring of vision.

Makeup on the surface of the eye or a reaction to solutions can vary from slight to marked redness of the eye with slight to marked pain.

Corneal abrasions (scratches) are usually quite painful, with or without the lens in the eye, and are associated with light sensitivity and redness. Vision may or may not be blurry.

Infections are a major concern because they may cause severe eye damage.

Simple pinkeye (a mild infection) is usually associated with redness, clear or mucous discharge, and matting of the eyelids. Vision is usually clear. When this happens, remove the contact lens in hopes of avoiding spread of the infection to the eye. Contact your fitter as to how to proceed.

Infections of the cornea are of more concern. These infections cause marked redness, light sensitivity, blurred vision, and a variable degree of pain. Occasionally, it is possible for the lens wearer to see the infection, which may appear as a white spot on the surface of the eye. These infections require immediate evaluation and care. Infections may be secondary to contaminated solutions, poor hygiene, contaminated tap water, or dirty contact-lens cases.

When a lens slips off the surface of the eye, vision immediately becomes blurry, which may or may not be associated with a sensation of something under the upper eyelid. If you can't move the lens to the correct position on the eye, you might have to seek the advice of your fitter.