There is a clear link between taking antibiotics and an increased risk of developing colon cancer within the next five to ten years, according to scientists at Umeå University, Sweden, after a study of 40,000 cancer cases. The impact of antibiotics on the intestinal microbiome is thought to lie behind the increased risk of cancer, they added.

“The results underline the fact that there are many reasons to be restrictive with antibiotics. While in many cases antibiotic therapy is necessary and saves lives, in the event of less serious ailments that can be expected to heal anyway, caution should be exercised. Above all to prevent bacteria from developing resistance but, as this study shows, also because antibiotics may increase the risk of future colon cancer,” explained Sophia Harlid, PhD, cancer researcher at Umeå, whose team published the study (“Antibiotics Use and Subsequent Risk of Colorectal Cancer: A Swedish Nationwide Population-Based Study”) in the Journal of the National Cancer Institute.

Researchers found that both women and men who took antibiotics for over six months ran a 17% greater risk of developing cancer in the ascending colon than those who were not prescribed any antibiotics. However, no increased risk was found for cancer in the descending colon. Nor was there an increased risk of rectal cancer in men taking antibiotics, while women taking antibiotics had a slightly reduced incidence of rectal cancer.

The increased risk of colon cancer was visible already five to ten years after taking antibiotics. Although the increase in risk was greatest for those taking most antibiotics, it was also possible to observe an admittedly small, but statistically significant, increase in the risk of cancer after a single course of antibiotics.

“Antibiotics use may increase colorectal cancer (CRC) risk by altering the gut microbiota, with suggestive evidence reported. Our study aims to investigate antibiotics use in relation to subsequent CRC risk, the investigators wrote.

“This is a nationwide, population-based study with a matched case-control design (first primary CRC cases and five matched, cancer-free controls). Complete-population data, extracted from Swedish national registers for the period 2005–2016, were used to calculate odds ratios and 95% confidence intervals.

“We included 40,545 CRC cases and 202,720 controls. Using the full dataset, we found a positive association between more frequent antibiotics use and CRC, excluding antibiotics prescribed within two years of diagnosis attenuated results toward the null. In site-specific analyses, excluding the two-year washout, the positive association was confined to the proximal colon (adjusted odds ratio for very high use vs no use = 1.17, 95% confidence interval = 1.05 to 1.31). For rectal cancer, an inverse association, which appears to be driven by women, was observed.

“Quinolones and sulfonamides and/or trimethoprims were positively associated with proximal colon cancer, whereas a more general inverse association, across antibiotics classes, was observed for rectal cancer. We found no association between methenamine hippurate, a urinary tract antiseptic not affecting the gut microbiota, and CRC risk.

“This register-based study covering the entire population of Sweden found a robust association between antibiotics use and higher risk of proximal colon cancer and an inverse association with rectal cancer in women. This study strengthens the evidence from previous investigations and adds important insight into site-specific colorectal carcinogenesis.”

To understand how antibiotics increase the risk, the researchers also studied a non-antibiotic bactericidal drug used against urinary infections that does not affect the microbiome. There was no difference in the frequency of colon cancer in those who used this drug, suggesting that it is the impact of antibiotics on the microbiome that increases the risk of cancer. While the study only covers orally administered antibiotics, even intravenous antibiotics may affect the gut microbiota in the intestinal system.

“There is absolutely no cause for alarm simply because you have taken antibiotics,” noted Harlid. “The increase in risk is moderate and the effect on the absolute risk to the individual is fairly small. Sweden is also in the process of introducing routine screening for colorectal cancer. Like any other screening program, it is important to take part so that any cancer can be detected early or even prevented, as cancer precursors can sometimes be removed.”

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