Eye prescriptions change because your eyes are living structures that respond to age, genetics, lifestyle, and overall health. For most healthy adults, a prescription stays stable for one to three years, so if yours is shifting at every exam, something specific is driving it. The cause depends largely on your age and circumstances.
Age Is the Most Common Factor
Your age bracket determines the most likely explanation. Children and teenagers experience the fastest changes because their eyes are still growing. The eyeball physically elongates during childhood, which increases nearsightedness. Kids with one nearsighted parent are about twice as likely to develop myopia, and those with two nearsighted parents are roughly five times more likely. Annual prescription changes during the school years are normal and expected, which is why yearly eye exams are recommended for anyone between ages 6 and 20.
Most people reach prescription stability in their early to mid-20s, and things stay relatively constant until the early 40s. That’s when presbyopia begins. The lens inside your eye, which is soft and flexible in youth, gradually stiffens as its proteins cross-link and compact over decades. By around age 40, the lens has lost enough flexibility that it can no longer change shape well enough to focus on close objects. You’ll notice it first in dim lighting or after long stretches of reading. Presbyopia progresses through your 50s, which means your reading prescription will keep updating even if your distance vision stays put. After 60, both near and distance prescriptions can shift more rapidly again, and annual exams become important once more.
Screen Time and Near Work
Hours spent focused on screens, books, or other close-up tasks can temporarily or permanently shift your prescription. Holding work closer than about 30 centimeters (roughly 12 inches) for stretches longer than 30 minutes increases the risk of becoming more nearsighted, especially in younger people.
There’s also a short-term effect. Prolonged near focus can fatigue the muscle inside your eye that controls lens shape. When that muscle stays contracted too long, it can lock up in a kind of spasm, leaving the lens stuck in a near-focused position. This creates what’s called pseudomyopia, a temporary nearsightedness that can show up during an eye exam and make your prescription look worse than it actually is. If you went straight from a long work session to your appointment, the result may not reflect your true prescription. The fix is simple: regular breaks from close work and spending more time outdoors, where brighter light helps regulate eye growth. Research shows that more than 40 minutes of daily outdoor time meaningfully reduces the risk of worsening nearsightedness in children, likely because light intensities above 3,000 lux (typical of being outside, even on a cloudy day) slow the elongation of the eyeball.
Blood Sugar and Diabetes
If you have diabetes or prediabetes, blood sugar swings can change your prescription from week to week. Here’s the mechanism: when blood sugar runs high, excess glucose in the lens gets converted into a sugar alcohol that can’t easily escape. When blood sugar then drops, water rushes into the lens to balance the concentration difference. That extra water changes how the lens bends light, temporarily shifting your vision toward farsightedness. The lens doesn’t physically change shape in a way that’s visible on imaging. Instead, the internal hydration of the lens tissue alters its focusing power.
This is why eye doctors often advise people with unstable blood sugar to wait until their glucose has been well-controlled for several weeks before getting a new prescription. A glasses prescription written during a blood sugar spike or crash may be inaccurate once levels stabilize.
Early Cataracts
A gradually increasing nearsighted prescription in someone over 50 can be an early sign of cataracts, specifically the type called nuclear cataracts that form in the center of the lens. As the lens core becomes denser and cloudier, it bends light more strongly, creating a myopic shift. Some people actually experience a brief period where their reading vision improves without glasses, sometimes called “second sight of the elderly,” because the added focusing power compensates for presbyopia. Distance vision worsens at the same time, though. If your nearsightedness has been creeping up in middle age or beyond, a cataract evaluation is worth discussing at your next exam.
Keratoconus and Corneal Changes
Rapidly changing astigmatism, particularly in your teens or 20s, can signal keratoconus. In this condition, the cornea (the clear front surface of the eye) progressively thins and bulges into a cone shape instead of staying evenly rounded. The result is increasingly distorted vision, light sensitivity, glare problems (especially while driving at night), and a need for frequent prescription updates that never quite seem to fully correct your sight. Keratoconus is one of the more important diagnoses to catch early because treatment can slow or halt the progression before vision loss becomes severe. If your astigmatism component keeps climbing, ask your eye doctor whether corneal mapping is appropriate.
Medications That Affect Vision
Several common medication classes can nudge your prescription. A large population study found that pain medications, including over-the-counter options like ibuprofen and acetaminophen as well as prescription pain drugs, were associated with a farsighted shift of roughly 0.7 to 1.15 diopters. Oral diabetes medications like metformin were linked to a mild nearsighted shift. Glaucoma eye drops showed the strongest associations, with some users measuring 1 to 2 diopters more nearsighted than expected.
These shifts don’t happen overnight and aren’t guaranteed for every person on these medications, but if you’ve started or changed a medication and your prescription seems off, it’s worth mentioning to your eye doctor.
Hormonal Changes
Pregnancy, breastfeeding, and hormonal medications can potentially affect vision. The cornea has receptors for estrogen, progesterone, and androgens, meaning hormone fluctuations can theoretically alter corneal thickness and curvature. In practice, research results are mixed. Some studies find measurable corneal changes during pregnancy while others find none. Many women do notice vision fluctuations during pregnancy, but these shifts are typically mild and often reverse after delivery or after stopping breastfeeding. Most eye care providers recommend waiting a few months postpartum before updating a prescription.
When Changes Are a Warning Sign
Gradual prescription changes over months or years are usually benign. Sudden changes are different. If your vision blurs, distorts, or worsens over hours or days rather than months, that can indicate something urgent. A sudden burst of new floaters (spots or strands drifting across your vision), flashes of light that look like lightning streaks or camera flashes, or a shadow or curtain effect across part of your visual field can signal a retinal detachment, which requires immediate attention to prevent permanent vision loss. Sudden blurriness where straight lines appear wavy also warrants a prompt evaluation.
The distinction that matters is speed. A prescription that drifts by a quarter or half diopter over a year or two is the normal rhythm of a living eye. A noticeable vision change over days, or one that doesn’t correct well with new lenses, deserves a closer look for an underlying cause.