More than one million women are diagnosed with breast cancer each year, making it the most common cancer among women worldwide. There are several established risk factors for breast cancer, including age, family history, fewer childbirths, earlier age at menarche, alcohol use, body mass index, and use of hormone therapy.

There is some limited evidence that antibiotic use may also be a risk factor for cancer. But until now, only one study has linked antibiotic use with breast cancer risk. These findings were limited because the study did not take into account length of antibiotic use, the type of antibiotic used, or conditions other than urinary tract infections.

A new study in the February 18, 2004 issue of the Journal of the American Medical Association linked antibiotic use with an increased risk of developing breast cancer and breast cancer mortality. It's important to keep in mind, however, that these findings are only preliminary and do not necessarily indicate that antibiotics cause cancer.

About the Study

The researchers looked at the data of 10,219 women who were at least 19 years old and were enrolled at Group Health Cooperative, a nonprofit health plan in Washington State. Among these women, 2,266 had breast cancer and 7,953 did not.

The researchers examined the pharmacy records of each participant as far back as 1977 and recorded antibiotic use and breast cancer incidence. From this data, the researchers determined whether antibiotic use was associated with breast cancer incidence or mortality.

The Findings

The researchers found that increasing cumulative days of antibiotic use and increasing number of antibiotic prescriptions were associated with increased risk of breast cancer. Additionally, increasing cumulative days of antibiotic use was associated with death due to breast cancer.

To decrease the likelihood of other factors affecting their results, the researchers also obtained information on known or suspected risk factors for breast cancer (i.e., age, menopausal status, use of hormone therapy).

While these findings may lead medical scientists to new studies assessing breast cancer risk, this study has very significant limitations:

  • Although the researchers adjusted for factors that may have caused an increase in breast cancer risk other than antibiotic use, much of this data was missing, limiting the validity of the adjustment.
  • Only 42% of the participants in the control group had received a mammogram within two years of their enrollment into the study, so there may have been participants in the control group with undiagnosed cancer.
  • This study only provides evidence for an association between antibiotic prescription and breast cancer. Women with more symptoms of infection may be at higher breast cancer risk for a variety of reasons. These reasons include: factors related to the infections themselves, an underlying disorder of immunity leading to frequent infections and higher cancer risk, or some effect of the antibiotics.
  • The relationship between breast cancer and antibiotic use was observed with many types of antibiotics, each very different from one another. This characteristic complicates any attempt to explain how the antibiotics might have acted to cause the cancer.

How Does This Affect You?

These findings are preliminary and, more than anything else, indicate the need for further studies to determine whether or not there actually is a link between antibiotic use and breast cancer.

The authors of an accompanying editorial point out that this study may raise many more questions than it answers, including:

  • Are there unmeasured factors (such as diet, alcohol use, or exercise) that may have increased breast cancer risk?
  • Are some of the diseases for which women take long-term antibiotics (acne, rosacea, chronic lung infections) themselves risk factors for breast cancer?
  • Are women who get more health care services (such as antibiotics) also more likely to have breast cancer detected by screening procedures such as mammography?

Understanding the relationship between antibiotic use and breast cancer is extremely important given both the number of women who are affected by breast cancer, along with the overuse of antibiotics. The overuse of antibiotics (a concern because it leads to antibiotic resistant bacteria) is all too common in the US, and scientists have spent years urging practitioners and the public alike to reduce antibiotic use.

The results of this study do not mean that women should stop taking antibiotics when prescribed by their physician. But if you have other risk factors for breast cancer and are frequently on antibiotics, you may want to discuss these findings with your physician.

Until there is further research directly linking antibiotic use with breast cancer, women should focus on modifying established risk factors for breast cancer such as:

  • Obesity after menopause
  • Physical inactivity
  • Alcoholic beverage consumption

Perhaps the most important “take home” message is that nearly half of the “controls” in this study had not been screened for breast cancer in the previous two years. If you are over 40 and haven't had a mammogram in the last one to two years, you too are overdue!