Dear Dr. Rx: Antibiotics are great, but now I keep hearing that you can become immune if you take them too often. Is this true if you’re taking antibiotics for different conditions or only if, say, you take them for the same kind of infection over and over? I’m thinking of urinary tract infections and bronchitis. Or does it have more to do with the amount you take, rather than the frequency?

To answer your question, it will help to clarify a few important points.

First, you do not become immune to the beneficial effects of antibiotics. Rather, the bacteria that cause infections can become immune to the antibiotic. We usually say that the bacteria become "resistant" to the antibiotic.

Second, the bacteria become resistant because of exposure to antibiotics. However, the word "antibiotic" is not very precise. Specifically, bacteria become resistant to antibacterial types of antibiotic drugs, such as penicillin and levofloxacin (Levaquin), as opposed to drugs used to treat viruses or fungal infections.

What this means is that you can become infected by a resistant bacteria because either you have taken antibiotics and then you become infected by your own resistant bacteria… or you can become infected by a resistant bacteria because someone else used antibiotics and those resistant bacteria then infected you. It is even possible that you will become infected by a resistant bacterium because of antibiotics that were used in food animals to promote animal growth.

Third, most bacteria that cause infections in humans have learned to become resistant to many of the antibiotics that have been commonly used in the 70 years since penicillin was first used to treat infections. This means when you become infected by a bacterium—for example, one that causes a urinary tract infection—there is an increasing likelihood that it will be resistant to the antibiotics that would normally be used.

Finally, there are two problems with the use of antibacterial antibiotics to treat diseases that are not usually caused by bacteria, such as bronchitis. It is ineffective because antibacterial drugs are ineffective for virus infections, which are the usual cause of bronchitis. Also, the normal bacteria that are exposed to the antibacterial drug--those that reside in the gastrointestinal tract, for example—can become resistant to the drug during treatment for bronchitis.

This, in turn, increases the probability that when you develop a urinary tract infection, it will be caused by a resistant bacterium because of the antibacterial drug used to treat a virus infection even weeks or months earlier.

The probability that you will become infected with a bacterium resistant to the antibacterial drug that would normally be used to treat your infection is related both to the amount and frequency of your previous exposure to antibacterial antibiotics… and the amount and frequency that others were exposed …and possibly the amount used in the foods that you eat.

It is generally believed that the solution to this problem will require a massive reduction in the use of antibacterial drugs from all sources, and even then it is not certain that rates of resistance will go down. Among infectious-disease specialists, this problem is regarded as one of the most important of our day. FP