JEFFREY BROWN: Breast cancer is the second leading cause of cancer deaths among women in the United States, and the most common cancer in women worldwide. The use and, according to many experts, overuse of antibiotics is widespread in many countries.
A study reported in this week's "Journal of the American Medical Association" makes a provocative link between the two. But it also raises many caveats and questions.
In the studies, researchers reviewed medical information of a group of more than 10,000 women in Washington State. They compared those in the group with breast cancer and those without the disease, and found that women with increased antibiotic use appeared to have a greater risk of breast cancer.
To tell us more about this study, we're joined by Dr. Roberta Ness, an epidemiologist at the University of Pittsburgh. She co-authored an editorial that accompanies the new report.
Dr. Ness, welcome. I want to get to the caveats in a moment, but first the study itself. Researchers are carefully saying that an association has been established. Can you tell us what that means?
DR. ROBERTA NESS: In this case, it means that the women who developed breast cancer were more likely to be long-term chronic users of antibiotics.
JEFFREY BROWN: Now they looked at a group of 10,000 women looking backwards at their records. Explain how they did that, please?
DR. ROBERTA NESS: Right. Well, this study was done in a health maintenance organization, and they had long-term pharmacy records. So in many cases, they had as much as 17 years of previous records of medication use. And they could compare amongst the women who developed breast cancer verses the women who did not develop breast cancer, those pharmacy records.
JEFFREY BROWN: Researchers said that it did not matter what type of antibiotic was used and what it was prescribed for. Is that correct?
DR. ROBERTA NESS: That is correct. They looked at a whole series of different classes of antibiotics ones that women are very familiar with, such penicillin, Tetracycline, sulfa drugs. And independent of the type of antibiotic used, there was a two-fold risk of breast cancer.
Furthermore they looked at a series of infectious diseases that would have warranted long-term antibiotic use such as chronic skin conditions -- acne for instance -- and chronic lung conditions. And in those cases again, there was about a two fold excess risk of breast cancer.
JEFFREY BROWN: Now they also tried very hard to rule out other potential risk factors as well.
DR. ROBERTA NESS: Yes, they did. After they had adjusted for a variety of risk factors that are known to increase the risk or decrease the risk of breast cancer, they still found this association.
JEFFREY BROWN: So if a real link is proven, what would be the significance of that?
DR. ROBERTA NESS: Well, I guess it would be quite substantial if it's real. Probably the important thing to say is that we know that antibiotics are a double-edged sword. The appropriate use of antibiotics can certainly be lifesaving, so use of antibiotics for bacterial infections is lifesaving in many cases. But the overuse of antibiotics, we know can cause harm.
There is a very well known association between the overuse of antibiotics and the development of resistant bacteria. There is some bacteria now that are in hospitals that are resistant to every antibiotic known to man. That's terribly worrisome.
So certainly at least what this means is that women should be cautious about the use of antibiotics for conditions in which they're not warranted, such as viral infections.
JEFFREY BROWN: Well let's look at the study and look at some of the caveats that were raised; chiefly, the lack of the causal link between the two. Explain that for us.
DR. ROBERTA NESS: That's right. Well, there's a difference between finding an association and saying that something causes another thing. Scientists require a great deal of evidence before they can say that something is causal. So, for instance, one of the very important things is that there be a consistent literature. At this point, this is really the first large study to show this association.
Certainly there will need to be a large body of evidence so many more studies conducted in a variety of ways which all show the same association. So that's one thing. The second thing is that we need to make sure that there is, what we call a temporal relationship between the antibiotic use and the development of breast cancer.
Because of the way the study was conducted, which is to say that it was a retrospective study, it was a study where they asked women or they looked back in time for the use of antibiotics, they can't be absolutely sure that the antibiotics preceded the breast cancer. And then finally, we require that there be a reasonable biologic mechanism and that's something that's in question in this study.
JEFFREY BROWN: You also raised in your editorial the possibility that it's not... that the cancer risk comes not from the antibiotics but from the underlying things that the antibiotics are intended to deal with: Infections, for example, or inflammation.
DR. ROBERTA NESS: That's right. The author suggested that the biologic mechanism might be that the antibiotics were interfering with the normal bacteria or flora that resides in the gut. And that bacteria is necessary for the absorption of the good nutrients and such that may actually prevent breast cancer. But we thought that that was somewhat less likely because all different classes of antibiotics had the same impact on breast cancer risk. And these different classes of antibiotics have very different effects on gut flora and furthermore, they act by very different mechanisms.
So it is rather odd that all of them have exactly the same effect. What we thought was perhaps the more likely explanation is that the antibiotics themselves are not what's the problem here, but in fact they're marking the reason why women take antibiotics and that is infection. Chronic infection causes chronic inflammation and we know that inflammation causes a series of different cancers.
There is a well known link between chronic inflammation and liver cancer. Gastric cancer, colon cancer, and in fact, it's been shown that non-steroidal anti-inflammatory medicine such as aspirin can reduce the risk of breast cancer. So it's been suggested for breast cancer as well chronic inflammation may play a role.
JEFFREY BROWN: So no one is saying now that women should stop taking antibiotics. What should they do?
DR. ROBERTA NESS: I think that they should view this with caution but interest. I think this is a great story to follow, but I certainly don't think that anyone should have a major change in their lifestyle as a result of this first finding.
JEFFREY BROWN: Okay Dr. Roberta Ness, thank you for joining us.
DR. ROBERTA NESS: Thank you.