Q. My vision has been playing tricks on me lately and I wonder what might be causing this. Several months ago I got a new prescription for glasses. Now I feel as if I need new glasses again.

What could cause my vision to change so soon?

A. Diabetes and pregnancy are two common causes of fluctuations in your vision.

High levels of blood sugar in diabetes can make the lens of your eye swell, causing your vision to go out of focus. That's why it's important for people with diabetes to have their blood sugar levels fairly stable before getting tested for a new pair of glasses.

Said another way, if you're diabetic and your vision changes, it's important to see your doctor to get your blood sugar checked and to make sure no other eye problems are developing.

During the last few months of pregnancy, the lens of the eye -- like other tissues in the body -- can swell, causing annoying changes in vision. So if you're pregnant, it's a good idea to wait until after delivery before getting a new prescription for eyeglasses or contact lenses.

Various eye disorders will interfere with clear vision, but these usually give you other symptoms as well. Eye inflammation or infection, such as iritis and conjunctivitis (pink eye), can dim your vision or produce enough mucus and discharge to temporarily blur your eyesight. These conditions will usually call attention to themselves by causing eye pain or redness.

Papillitis -- inflammation of the nerves of vision in back of the eye -- can cause temporary loss of vision and pain behind the eye.

Sudden change of vision can be a sign of something serious, and calls for a prompt visit to your doctor.

Q. I am a healthy 41-year-old man with good blood pressure, although my parents had heart problems. At my latest medical exam, my doctor suggested I take an aspirin a day as a preventive measure against heart disease. I recently saw an aspirin commercial on television suggesting the same thing.

At my age, assuming I am not

allergic to aspirin, is "an aspirin a day" a good rule to follow?

A. Although some doctors advise taking aspirin as a preventive step, I would stop short of recommending it for everyone.

Because aspirin helps thin the blood, it's beneficial in people with hardening of the coronary arteries around the heart and the arteries leading to the brain. Studies show that aspirin cuts the risk of heart attack in people with worsening chest pain caused by poor blood flow in the coronary arteries (angina). Other studies show that aspirin reduces the chances of stroke in people who have brief episodes of poor blood flow to the brain (transient ischemic attacks).

Although it's tempting to try aspirin before warning signs of stroke or heart attack develop, so far we don't have any proof that this treatment works. Right now there's a study in progress -- using American doctors as the study subjects -- to see whether taking aspirin can reduce your risk of heart attack. The results should shed light on this question, and may lead to an important advance in preventive medical practice.

In the meantime, it's difficult to say whether the risks of aspirin therapy -- including intestinal bleeding and allergic reactions like asthma -- outweigh the potential benefits when used to try to prevent heart disease. Another undecided question is what dose of aspirin works best, if any. Doctors have recommended doses ranging from one baby aspirin a week to one regular adult aspirin every day.

In sum, while there is evidence that aspirin can be of benefit, I wouldn't recommend an aspirin a day for everyone until more studies are done. However, that's not to say your doctor's recommendation is without some merit.

Jay Siwek, a family physician from Georgetown University, practices at the Fort Lincoln Family Medicine Center and Providence Hospital in Northeast Washington.

Send questions to Consultation, Health Section, The Washington Post, 1150 15th St. NW, Washington, D.C. 20071. Questions cannot be answered individually.