In a new study, researchers found antibiotic use (pills/capsules) is linked to a heightened risk of bowel (colon) cancer, but a lower risk of rectal cancer.
They also found the cancer risk depends on the type and class of drug prescribed.
The research was conducted by a team from Johns Hopkins University.
In 2010, patients around the world took an estimated 70 billion doses of antibiotics—equivalent to 10 doses each.
Antibiotics have a strong and long-lasting impact on the gut microbiome, altering the balance of helpful and harmful bacteria.
In the study, the researchers wanted to find out if this might affect bowel and rectal cancer risk, and how.
They drew on data submitted to the nationally representative Clinical Practice Research Datalink (CPRD) between 1989 and 2012.
This contains the medical records of around 11.3 million people from 674 general practices—around 7% of the UK population.
The researchers collected prescribing information for 28,930 patients diagnosed with bowel (19,726 ) and rectal (9254) cancers during an average monitoring period of 8 years, and for 137,077 patients, matched for age and sex, who didn’t develop these cancers.
They found antibiotics had been prescribed to 70% (20,278) of patients with bowel and rectal cancers and to 68.5% (93,862) of those without.
Nearly six out of 10 study participants had been prescribed more than one class of antibiotic.
The team found people with bowel cancer were more likely to have been prescribed antibiotics.
The link between bowel cancer and antibiotic use was evident among patients who had taken these drugs more than 10 years before their cancer was diagnosed.
Patients who developed bowel cancer were more likely to have been prescribed antibiotics targeting anaerobes, which don’t need oxygen, as well as those targeting aerobes, which do—than patients without cancer.
But patients with rectal cancer were less likely to have been prescribed antibiotics targeting aerobic bacteria.
The team found the cancer site was also linked to antibiotic use.
Cancer of the proximal colon—the first and middle parts of the bowel—was associated with the use of antibiotics targeting anaerobes, when compared to people without cancer.
But antibiotic use was not linked to cancer of the distal colon—the last part of the bowel.
After taking account of potentially influential factors, the team found the use of antibiotics for a short period (16+ days) was linked to a higher risk of bowel cancer, with the impact strongest for cancers of the proximal colon.
The reverse was true for rectal cancers, where antibiotic use exceeding 60 days was associated with a 15% lower risk compared with no use.
The findings suggest a pattern of risk that may be linked to differences in gut microbiome (bacteria) activity along the length of the bowel.
The team also found penicillins were consistently associated with a heightened risk of bowel cancer of the proximal colon.
Ampicillin/amoxicillin was the penicillin most commonly prescribed to these patients.
By contrast, the lower risk of rectal cancer was associated with prescriptions of tetracyclines.
The team says future work needs to capture lifestyle factors and hospital treatment, which may have affected overall cancer risk.
The study is published in the journal Gut.
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