Antibiotics Use and Subsequent Risk of Colorectal Cancer: A Swedish Nationwide Population-Based Study.

Background 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. Methods This is a nationwide, population-based study with a matched case-control design (first primary CRC cases and 5 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. Results 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 2 years of diagnosis attenuated results toward the null. In site-specific analyses, excluding the 2-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. Conclusions 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.