DR. K: Do reading glasses make eyesight worse?

DEAR DOCTOR K: I am starting to have trouble with my vision, especially when I'm reading. I just can't seem to focus on the words as well as I once did. But I've heard that wearing glasses to help me read will make my eyesight worse. Is that true?

DEAR READER: You can rest easy. Glasses won't make your eyesight worse. They will make it easier for you to enjoy reading, though.

Many people start having trouble reading in their mid-40s. That's because of a condition called presbyopia (prez-bee-OH-pee-ah), in which the lens of the eye has more trouble focusing on things. The closer the object, the more the lens has to flex so that you can see it clearly. With presbyopia, the lens slowly grows larger, thicker and less flexible.

I often tell patients to try this simple test. Hold a book about 6 inches in front of one eye (use a ruler). Odds are good that the print is blurry. That's because your lens can't flex enough.

You have several options for correcting presbyopia. The most common remedy is reading glasses. If you already wear glasses to see things in the distance more clearly, talk to an optometrist or ophthalmologist about bifocals, trifocals or progressive lenses. All of these types of glasses work for both distance vision and reading.

You can also get prescription contact lenses that correct the vision in one eye for reading and in the other for distance. This technique is called monovision. Multifocal contact lenses are another choice. Like bifocals, trifocals and progressive lenses, these contact lenses can improve both your distance vision and near vision.

Drugstores and many supermarkets now sell off-the-rack reading glasses at different magnifications. Some look quite jazzy, and they tend to be less expensive than prescription glasses. So my patients often ask me what's wrong with just buying them, instead of seeing an eye doctor. I tell them that sometimes the correction that seems to work in the drug store isn't really right, and that can strain your eyes.

There's an even more important reason to have regular eye exams. Eye conditions such as glaucoma can damage your eyes without your noticing -- until it's too late to avoid permanent damage. Ask your doctor how often you should see an ophthalmologist or other eye care professional. The answer varies depending on your age, health, vision problems and family history.

Also, any person's vision is likely to change over time. Usually the changes are subtle, and you don't notice them at first. For example, I wear reading glasses, and last week I noticed that I wasn't seeing as clearly out of my right eye. The left was fine. And when my glasses were checked 18 months ago, my right eye was fine, too. But not anymore. I'm seeing the eye doctor next week. Maybe the lens in my right eye has gotten stiffer, or developed cataracts. The only way to know the problem, and fix it, is to get an eye exam.

• Dr. Komaroff is a physician and professor at Harvard Medical School. Go to his website to send questions and get additional information: www.AskDoctorK.com.