Q. I have a heart murmur and am scheduled for some dental work. My dentist wants me to take antibiotics (three grams of penicillin over a six-hour period) to prevent a possible heart infection. Is it wise to take so much medicine all at once, and could a possible side effect be to make me resistant to penicillin in the future?

A. People with various kinds of heart conditions are at risk of developing infections of one of the four heart valves, delicate structures that control the flow of blood in the heart. Procedures that damage surfaces inside the body often let bacteria into the bloodstream, where they can travel to the heart and set up an infection, particularly on valves that are already defective.

Heart valve infections are known as endocarditis (endo refers to the inside of the heart, and carditis means heart inflammation). Endocarditis is a serious, often life-threatening infection. Treatment requires getting intravenous antibiotics for long periods, up to 12 weeks, and sometimes open heart surgery to replace an infected valve with an artificial one.

The idea behind taking antibiotics before dental procedures is that you would kill any bacteria that might happen to get into your bloodstream before they could cause an infection.

People with artificial heart valves, most congenital heart defects, valves damaged by heart disease and people who have previously had endocarditis are among those who should take preventive antibiotics. People who have had bypass surgery, however, do not need preventive therapy.

Some of the procedures that can let bacteria enter the bloodstream are dental work that causes your gums to bleed; having your tonsils out; bladder, prostate or intestinal surgery; biopsies of the lung or intestines; and gallbladder surgery. Because the list is longer and occasionally changes, the easiest thing to do if you have a heart condition is ask your doctor whether you need preventive antibiotics before any operation or special procedure you have.

In 1984 the American Heart Association updated its recommendations about preventive antibiotics before dental procedures for people with heart conditions. To simplify things, the length of treatment was shortened from two days to six hours in most cases. Although the dose is high (two grams of penicillin one hour before, and one gram six hours later for certain dental procedures), it's virtually as safe as smaller doses. Taking this much penicillin, even for repeated treatments, doesn't lessen its effectiveness for future use.

For people with artificial heart valves and other high risk conditions, good dental hygiene, especially prevention of gum disease by regular flossing, may be more important than preventive antibiotics before dental procedures.

*Q. I read your answer about ear wax and how cotton swabs shouldn't be used to clean the ear canal. Before, I always cleaned my ears with them. Whenever I did, it made me cough or gave me a tickling sensation in my throat. Can you tell me what causes this and whether I should be concerned?

A. You've discovered an interesting reaction that some people experience when touching their ear canals. While your response doesn't point to anything to be concerned about, it does show what intricate connections our bodies have.

There are nerves in your head that send branches to your ears and throat, among other areas. Stimulation of this nerve by touching your ear canal will sometimes trigger a cough, sneeze or tickling sensation in your throat. Doctors sometimes inadvertently cause this reaction when they examine your ears with an otoscope.

This nerve connection also explains why some people with a sore throat feel pain in their ears even when they don't have an ear infection, and vice versa. Because of this, doctors routinely examine both areas when a patient has symptoms in either.

Q. During a routine physical, my urine test showed a trace of blood. To make sure the lab report was correct, my doctor repeated the test and found no blood. I went back for another urine test, which was okay. My doctor wants me to come back for a fourth test, but I'm a little worried about the discrepancy, because I know blood in the urine can be very serious.

A. You didn't mention your age or sex, two things that can make a lot of difference in answering your question, but I think I can give you some general guidelines about how doctors use tests to uncover hidden disease.

Assuming you're otherwise healthy and don't have any symptoms of something wrong with your kidneys or bladder, the question is what to make of the first test that showed blood in the urine.

The first possibility is that it was a false result, an error in testing. This happens more often than most people realize. Doctors consider a test pretty good if it's accurate 95 percent of the time. This means that one time in 20 the test is wrong, either falsely positive or falsely negative.

Because all tests have an inherent degree of error in them, doctors often will double-check a test before embarking on a costly, anxiety-producing, and sometimes risky search for its cause. Part of your doctor's job is weighing the risk of missing a serious cause of a symptom or abnormal test against the cost -- financial and otherwise -- of evaluating its significance.

Another explanation for the first test showing blood is that you had a relatively harmless condition causing it. For example, many women will test positive for blood in the urine if the specimen was taken close to the time of their menstrual period.

If you did have a serious condition -- bladder cancer, for example -- it's likely that the urine would continue to show blood on subsequent tests. In general, the more evidence that something serious is present, the more your doctor will press his investigation to uncover its cause.