After reviewing a number of published studies, researchers from the University of Athens in Greece concluded that antibiotics use appears to be linked to a slightly higher risk of breast cancer. While this is not the first time such a link has been investigated, their study also tried to address some of the problems behind the inconsistent results of previous studies.

You can read about the work that led to this conclusion in a recent study published in the journal Pharmacoepidemiology and Drug Safety: an early edition was made available online on 15 September.

First author Dr Theodoros N. Sergentanis and colleagues from the University of Athens conducted a meta-analysis: they pooled results from previous studies and the re-analyzed them as if they came from one large study.

They did two types of analysis: one that compared the effect on breast cancer risk of ever using antibiotics versus never using them, and another that took the amount of antibiotic use (in terms of numbers of prescriptions) into account.

They did this because previous studies have yielded conflicting results, and they lacked consistency in measuring antibiotic use.

For their study, Sergentanis and colleagues searched the MEDLINE, Cochrane, and EMBASE databases for eligible studies published up to July 2009.

They found 5 case-control studies that met their criteria for being able to compare ever-use versus never-use. These covered a total of 13,069 cases and 73,920 controls.

After pooling the results, they analyzed the Odds Ratio (OR) of ever-use versus never-use and found that having ever used antibiotics was linked with a slightly elevated risk of developing breast cancer (pooled OR = 1.175, 95% confidence interval 0.994 – 1.387).

When they did a meta-regression analysis to look at what happened to cancer risk with varying rates of antibiotic prescriptions, they found a “borderline dose-response effect implicating the number of antibiotic prescriptions”.

They concluded that:

Antibiotic use seems associated with slightly elevated breast cancer risk.”

However, they noted that the underlying nature of the link “remains elusive”, and it could be due to secondary associations not addressed in the studies: these could be directly causing the effect, or just confounding it.

But whatever the underlying reasons, they said this link should be investigated further because of the “potentially important public health implications”.

One explanation that has been offered before is that using antibiotics weakens the immune response against cancer.

Other explanations suggest that antibiotics kill gut-friendly bacteria that help digest food and nutrients that protect against cancer, or that it is the conditions that lead to the use of antibiotics that cause the increased risk of breast cancer.

Or it could be a combination of these.

The fact that the authors only found a borderline effect when they looked at the dose-response relationship between amount of antibiotic use and breast cancer risk is not very surprising, because counting number of prescriptions, as opposed to for example, differentiating between low-dose over a long time and high-doses once in a while, may smooth out some subtle but significant effects.

Several years ago, because of concerns over the public health implications of misuse of antibiotics, and the consequent increase in drug resistant strains of “superbugs”, the Centers for Disease Control and Prevention (CDC) in the US started a campaign called Get Smart: Know When Antibiotics Work.

Much of the misuse of antibiotics, which can only cure bacterial infections, arises because they are incorrectly prescribed for viral infections such as colds, flu, bronchitis, sore throats not caused by strep, and others.

Thus it would appear that the take home message from this study is much the same as that of another study published in JAMA in February 2004, that also found evidence that use of antibiotics is associated with an increased risk of breast cancer.

One of the investigators on the JAMA study, Dr Stephen H. Taplin, of the National Cancer Institute and formerly with the Group Health Cooperative (GHC) in Seattle, told the press that their results do not mean women should stop taking antibiotics for bacterial infections:

“Until we understand more about the association between antibiotics and cancer, people should take into account the substantial benefits that antibiotics can have, but should continue to use these medicines wisely,” he added.

“Is antibiotic use a risk factor for breast cancer? A meta-analysis.”
Theodoros N. Sergentanis, Flora Zagouri and George C. Zografos.
Pharmacoepidemiology and Drug Safety, first published online : 15 September 2010.
DOI: 10.1002/pds.1986

Additional sources: MNT Archives.

Written by: Catharine Paddock, PhD