New research has found that antibiotic use is linked to an increased risk of bowel (colon) cancer, but a lower risk of rectal cancer, suggesting that, to some extent, the risk of cancer may depend on the type and class of antibiotics prescribed.
Carried out by researchers at Johns Hopkins University School of Medicine, USA along with the University of East Anglia Norwich Medical School and Norfolk and Norwich University Hospital NHS Trust, UK, the new study set out to investigate how the strong and long-lasting impact of antibiotics on the gut microbiome, which is the balance of helpful and harmful bacteria in the gut, could affect bowel and rectal cancer risk.
To do so, the researchers looked at the medical records of 19,276 patients diagnosed with bowel cancer and 9,254 patients diagnosed with rectal cancers who were followed for at least two years. The researchers also looked at 137,077 patients, matched for age and sex, who didn’t develop these cancers.
The findings, published in the journal Gut, showed that after taking account of potentially influential factors, such as being overweight, smoking, and moderate to heavy drinking, cumulative use of antibiotics for a relatively short of time (16 or more days) was associated with an increased risk of bowel cancer compared to no antibiotic use, with the strongest association found for cancers of the proximal colon, which is the first and middle part of the bowel.
In contrast, antibiotic use for more than 60 days was actually associated with a 15% lower risk for rectal cancers, compared with no antibiotic use.
The researchers also found that the type of antibiotic prescribed appeared to affect the risk of cancer and the cancer site. For example, when looking at patients who had only been prescribed one class of antibiotic, as opposed to none, penicillins were consistently associated with an increased risk of bowel cancer of the proximal colon.
Ampicillin/amoxicillin was the penicillin most commonly prescribed to these patients.
The lower risk of rectal cancer was associated with prescriptions of tetracyclines.
The researchers note that as an observational study, the findings do not show cause and effect. Limitations of the study also include the fact that the researchers were unable to include any hospital treatment or influential lifestyle factors for all the participants, which may have affected overall cancer risk.
However, they add that the findings still suggest that antibiotics can have a varying effect on gut microbiome along the length of the bowel, and conclude that, “whether antibiotic exposure is causal or contributory to colon cancer risk, our results highlight the importance of judicious antibiotic use by clinicians.”
The researchers point out that in 2010, patients around the world took an estimated 70 billion doses of antibiotics – equivalent to 10 doses each.