Contact lens prescriptions expire due to a combination of medical monitoring and federal regulation. The expiration date ensures that patients receive regular eye health examinations, since contact lenses are classified as medical devices that sit directly on the eye. This requirement is reinforced by laws, such as the Fairness to Contact Lens Consumers Act (FCLCA) in the United States, which mandates a minimum one-year expiration date for prescriptions unless a medical reason calls for a shorter period. The yearly check-up protects the eye from potential long-term damage caused by lens wear.
The Difference: Contacts vs. Glasses Prescriptions
The fundamental reason contact lens prescriptions differ from those for eyeglasses is their physical placement on the eye. Eyeglasses sit approximately 12 millimeters away from the eye’s surface, requiring a different power calculation for stronger prescriptions, known as vertex distance compensation. Contact lenses, by contrast, rest directly on the cornea, which means they must be precisely fitted to the eye’s unique shape, not just its refractive error.
A contact lens prescription includes specific physical parameters that are absent from a glasses prescription. These parameters include the Base Curve (BC), which measures the lens’s curvature to match the cornea, and the Diameter (DIA), which defines the lens’s width for proper coverage. A poor fit, even with the correct power, can lead to discomfort, corneal damage, and serious eye health complications. The prescription also specifies the exact brand and material, as lens manufacturers use different technologies that affect oxygen flow and fit.
Protecting Eye Health: The Medical Rationale
Wearing contact lenses can induce subtle, long-term physiological changes in the eye that require professional monitoring. One significant concern is oxygen deprivation, or hypoxia, which can occur when the lens material reduces the amount of oxygen reaching the cornea. The cornea is an avascular tissue, meaning it receives oxygen primarily from the air and tears, and contact lenses naturally interfere with this process. Chronic hypoxia can trigger a condition called corneal neovascularization, where new, fragile blood vessels grow into the normally clear cornea.
These invading blood vessels can permanently cloud vision and are often difficult to detect without high-magnification instruments used during an eye exam. Contact lenses can also destabilize the tear film, the fluid layer that lubricates and protects the eye. Changes in tear film composition or a reduced blink rate due to lens wear can lead to dry eye symptoms and increase the risk of surface damage. The annual examination allows the eye doctor to assess the physical interaction between the lens and the eye, including checking for protein or lipid deposits that build up on the lens surface.
Mandated Expiration: The Regulatory Requirement
In the United States, the expiration date is legally enforced to ensure continuous medical supervision. The Fairness to Contact Lens Consumers Act (FCLCA) and the rules implemented by the Federal Trade Commission (FTC) mandate that prescribers must release a copy of the prescription to the patient. The law further specifies that a prescription must be valid for a minimum of one year, establishing the typical annual expiration cycle.
This regulatory framework is designed to protect consumers by preventing manufacturers and retailers from withholding prescriptions. The required expiration date acts as a mechanism to compel a necessary medical check-up before lenses can be repurchased. Sellers are prohibited from filling a prescription that has been verified as expired, inaccurate, or otherwise invalid by the prescriber.
Risks of Using Outdated Prescriptions
Ignoring the expiration date and continuing to wear lenses based on an old prescription carries several health risks. The most immediate danger is the increased potential for severe eye infections, such as microbial keratitis, an infection of the cornea caused by bacteria or fungi. An outdated prescription means a patient is likely using lenses that are past their manufacturer’s recommended replacement schedule, leading to material breakdown and a build-up of deposits.
These older lenses can become less permeable to oxygen and may harbor microbes, turning the lens into a source of infection. An ill-fitting lens, which can result from subtle shifts in corneal shape since the last exam, may cause abrasions or ulcers on the eye’s surface. Even if the lenses are new but the prescription is old, the incorrect base curve or diameter can cause the lens to fit too tightly or too loosely, leading to inflammation or chronic discomfort.