Q. Is it possible to develop sensitivity to penicillin drugs (like ampicillin) over time? I'm 32 years old and have had many bouts of tonsillitis. My doctor always treated me with penicillin in the past and I never had any problem with it.

During the past year, whenever I've taken any form of penicillin, I've suffered with side effects including stomach cramps, diarrhea and what seems to be a bowel infection.

Can I become allergic to penicillin after years of using it without any difficulties? Should I not take penicillin again in the future?

A. Your question really addresses several complications of using penicillin, or most antibiotics for that matter. I'll discuss these problems one by one.

First, let me make a distinction between an allergic reaction and an adverse reaction or side effect. Allergic reactions occur when your body's immune system goes awry and is tricked into reacting against a drug in a potentially harmful way. Only people whose immune system is "sensitive" to the drug will react, and they should ordinarily avoid taking it.

Adverse reactions, or side effects, are not an allergic reaction (they don't involve your immune system). They are caused by the drug affecting some part of your body in an undesirable way. These responses range from annoying (diarrhea or constipation) to serious (internal bleeding) and can affect anyone. In fact, most drugs will produce side effects in a certain percentage of people.

The reason for distinguishing between allergic and adverse reactions is that if you're allergic, you shouldn't take the drug, whereas if you have a side effect you may or may not be able to take it again depending on how serious a reaction you had.

In your case, your stomach symptoms are an adverse reaction, not an allergic one, so you needn't necessarily avoid penicillin in the future. Your reaction is probably caused by penicillin killing off the bacteria normally present in your intestines, allowing other bacteria to take over and cause an intestinal infection, something like traveler's diarrhea.

To answer your other question, you can become allergic to penicillin even after years of taking it without any problem. There are three types of reactions: immediate, delayed and late. Immediate reactions, which fortunately are uncommon, can be mild, like hives, or severe and life-threatening, like shock. Delayed reactions -- the most common allergic response -- include skin rashes and usually take a few days to develop. Late reactions, which are rare, can cause a generalized flu-like illness one or two weeks after a person receives penicillin.

People with other allergies, like hay fever, asthma and hives, are more likely to have an allergic reaction to penicillin. In any case, an injection is more likely than a pill to cause an immediate or serious reaction.

One last point is that ampicillin, unlike other penicillins, can cause a measles-like rash that isn't a true allergic reaction. If so, you're not prevented from taking other penicillins in the future.

Q. My sons have hemophilia. It is an incurable bleeding disorder, so anything new that may be of help is of vital interest. I've heard of a new blood-clotting product recently approved by the Food and Drug Administration, which my doctor knows little about. We are most anxious to know more and wondered if you could help.

A. You're probably referring to Cyklokapron (tranexamic acid), a drug used to stop bleeding after tooth extractions in people with hemophilia.

Hemophilia is an inherited bleeding disorder. Affected people lack one of several substances in the blood responsible for clotting. They suffer from internal bleeding and have difficulty in stopping bleeding, such as after having a tooth pulled. Although this isn't a serious problem for most people, those with hemophilia can lose significant amounts of blood after tooth extraction.

Hemophilia is treated with costly replacement of the missing clotting substance, called factors. Factor VIII replacement, the most common deficiency, can cost $20,000 or more per year. Because clotting factors are obtained from many units of donated blood, replacement therapy also carries the risk of hepatitis and AIDS, especially before the time when blood was screened for the AIDS virus.

Cyklokapron -- which promotes blood clotting -- is used along with replacement therapy during and after tooth extractions. Side effects include occasional nausea, vomiting or diarrhea, which seem to clear with lowered doses.

Eye damage has occurred in laboratory animals given this drug. No serious eye damage has been reported in humans, although some people have experienced temporary changes in vision, particularly color vision. Because changes in color vision may represent an early warning sign of eye damage, people who are color blind should not take this medication.

Cyklokapron is usually taken just for two or three days following tooth extraction. If it is used longer, you should be examined by an ophthalmologist before and while taking it. Some experts recommend that you have an eye exam beforehand to check your color vision.

For general information about hemophilia, contact the Washington chapter of the National Hemophilia Foundation, P.O. Box 2644, Fairfax, Va. 22031; 352-2144.

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.