Yes, a separate prescription is required to purchase contact lenses, as contacts are classified as medical devices. Unlike eyeglasses, which sit away from the eye’s surface, contact lenses rest directly on the cornea, making them a regulated medical product by the Food and Drug Administration (FDA). This distinction means that all contact lenses, including those used for purely cosmetic purposes, require a valid prescription and professional fitting to ensure ocular health. The prescription is necessary because a poorly fitting lens poses risks that go beyond simply blurry vision, potentially leading to serious eye complications. The unique environment of the eye demands specific measurements and material considerations that an eyeglass prescription does not provide.
Unique Measurements for Contact Lenses
An eyeglass prescription only specifies the refractive power needed to correct vision, which is insufficient for a contact lens. Contact lenses require additional, highly specific measurements to ensure the lens conforms precisely to the eye’s shape. The most important of these is the base curve (BC), which measures the curvature of the back surface in millimeters to match the curvature of the cornea. A typical base curve for soft lenses falls between 8.3 and 9.0 mm, with a smaller number indicating a steeper curvature.
The diameter (DIA) is also measured, representing the total width from edge to edge, typically ranging from 13.8 to 14.5 mm for soft lenses. These physical parameters determine how the lens rests on the eye; if the base curve is too flat or too steep, the lens can move improperly, causing discomfort, blurry vision, or even damage.
Lenses that fit too tightly can cut off the necessary oxygen supply to the cornea, a condition called hypoxia, while loose lenses can lead to corneal abrasions or fall out easily. The prescription must also specify the exact material, brand, and type of lens, as different manufacturers use varying materials. Finally, the power may need slight adjustment from the glasses prescription because the contact lens sits directly on the eye, eliminating the distance between the lens and the eye surface, a factor known as vertex distance.
The Contact Lens Fitting and Evaluation
Obtaining a contact lens prescription involves a specialized fitting and evaluation process that goes beyond a standard eye exam. This process begins with an assessment of the cornea’s curvature, often using an instrument like a keratometer or corneal topographer. The eye care professional also evaluates the quality and quantity of the patient’s tear film, as dry or unstable tears can significantly affect lens comfort and wearing time. These initial measurements help the doctor select appropriate trial lenses based on size, material, and power.
The patient is then given trial lenses to wear for a short period, allowing the doctor to observe the lens’s movement and fit on the eye. The professional checks for proper centration and adequate tear exchange under the lens, which is necessary for corneal health. Multiple follow-up visits may be required to ensure the lenses remain comfortable and the vision correction is stable after a period of initial wear. The fitting process also includes instruction on proper insertion, removal, and hygiene to prevent eye infections and other complications.
Prescription Expiration and Renewal
Contact lens prescriptions typically have an expiration date, which is most commonly set at one year. This duration is mandated by federal law, specifically the Fairness to Contact Lens Consumers Act (FCLCA), which requires a minimum one-year validity unless state law allows for a longer period. Since contact lenses rest on the cornea, they can induce changes like corneal warpage or reduced oxygen flow that may go unnoticed by the wearer but pose long-term risks.
The expiration date ensures that wearers must return for a re-examination, allowing the eye care professional to assess the cornea’s health and confirm the current lens parameters are still appropriate. If a prescription expires, a new eye exam and fitting evaluation are required before new lenses can be legally purchased. A doctor may set an expiration date shorter than one year for specific medical reasons, which must be documented in the patient’s record, but they cannot set a date shorter than the period they recommend for a medically necessary re-examination.