Why does my eye glasses/contact lens prescription expire?

According to All About Vision, your eyes can change over time, causing your current prescription to become inaccurate and no longer adequately correct your vision. Eyeglasses made with an out-of-date, inaccurate prescription can cause eye strain, headaches, and possibly a serious accident due to blurry vision. Even if your glasses prescription stays the same from exam to exam, you should have routine eye exams to make sure you’re not developing potentially sight-threatening eye conditions like diabetic eye disease, glaucoma, or macular degeneration.