Women with triple-negative breast cancer who received multiple antibiotic prescriptions within three years after their cancer diagnosis were more likely to experience disease recurrence and to die from their cancer than those who took fewer courses of the drugs, according to a study by researchers at Stanford Medicine. The effect on survival was not due to differences in cancer severity, the study showed.

The risk increased substantially with the total number and types of antibiotics prescribed for each patient.

“Each additional antibiotic increased the risk of death between 5% and 18% relative to patients who weren’t prescribed antibiotics,” said Julia Ransohoff, MD, a fellow in hematology and medical oncology at the Stanford School of Medicine. “It is important to interpret these findings with caution, however. We can’t let life-threatening infections go untreated. But this study suggests that we consider how best to treat them without raising the risk of cancer recurrence.”

Ransohoff is the lead author of the study, which was published online April 12 in Nature Communications. Allison Kurian, MD, professor of medicine and of epidemiology and population health, and Ami Bhatt, MD, PhD, associate professor of medicine and of genetics, are the senior authors of the study.

“These findings offer insight into the role of the immune system, and factors that may perturb its function, in fighting the most aggressive type of breast cancer,” Kurian said. “Gaining a better understanding of this process will be important to guide patient care.”

Effect on a type of immune cell

The researchers studied 772 women who were diagnosed between January 2000 and May 2014 and treated at Stanford Health Care or Palo Alto Medical Foundation. They found that treatment with antimicrobials (a class of drugs that includes antibiotics to treat bacterial infections and antifungals to treat fungal infections) was associated with a decrease in the numbers of an immune cell called lymphocytes circulating in a patient’s blood. Lymphocyte numbers have been shown to correlate with response to treatment and overall survival in people with breast cancer. (Although the researchers studied total antimicrobial prescriptions, 99% of the women in the study who were prescribed antimicrobials received antibiotics.)