Researchers examined data from Swedish national registers for 40,545 individuals diagnosed with colorectal cancers from 2005 to 2016 as well as 202,720 matched controls. They linked these records with data on antibiotic utilization during the same period from the Swedish Prescribed Drug Register.
Overall, people who had total antibiotic usage of six months or more were more likely to develop colorectal cancer (odds ratio 1.17) than those who had no antibiotic use, the study team reports in the Journal of the National Cancer Institute.
However, when researchers excluded any antibiotic usage within two years of a cancer diagnosis, this association was no longer statistically significant.
Researchers did additional analysis based on tumor site, and excluding antibiotics prescribed within two years of the cancer diagnosis. In these analyses, antibiotics use for more than six months total was significantly associated with proximal colon cancer (OR 1.17) but not with distal colon cancer or rectal cancer.
"We think this has to do with antibiotics disrupting the microbiome balance in the intestine," said senior study author Sophia Harlid, an associate professor in molecular epidemiology in the department of radiation sciences and oncology at Umea University in Sweden.
This may result in less diversity of bacterial species and increase the growth of some especially unfavorable bacteria, resulting in an environment with more inflammation and a higher likelihood of developing tumors, Harlid said by email.
"However, we do not yet know for a fact that it is the antibiotics that cause the risk increase, as our study can only show that there is a correlation between antibiotics intake and colon cancer risk," Harlid said.
During the study period, approximately 19% of the people who developed colorectal cancer and 22% of controls in the general population had no antibiotics prescriptions.
Among people who developed colorectal tumors, the mean age at diagnosis was 72 years. The most common type was proximal colon cancer (36%), followed by rectal cancer (33%), and distal colon cancer (29%).
One limitation of the study is that it only covered orally administered antibiotics, the study team notes. It's possible that intravenous antibiotics might also alter the gut microbiota they point out.
The study also doesn't prove antibiotics cause colorectal cancers, noted Dr. Andrew T. Chan, a professor at Harvard Medical School and a gastroenterologist at Massachusetts General Hospital in Boston who wasn't involved in the study.
"Even if this was cause and effect, the use of antibiotics likely only plays a small role among many risk factors for colorectal cancer," Dr. Chan said by email. "Thus, colorectal cancer risk should not be a reason to minimize the use of antibiotics if there is a strong clinical indication."
That said, the study should serve as a reminder for clinicians to use antibiotics only when indicated, Dr. Chan added.
"This potential association provides yet another reason to avoid using antibiotics indiscriminately," Dr. Chan said.
SOURCE: https://bit.ly/2Y5z5KU Journal of the National Cancer Institute, online September 1, 2021.
By Lisa Rapaport
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