Among 83 prescription drugs tested in 1,034 colonoscopy-referred adults aged 55–77, 38 altered gut microbial diversity and bacterial species abundance. Twelve years of national registry prescription records revealed six known colorectal cancer (CRC) biomarker candidates responding to specific drugs: Escherichia coli increased with metformin; Clostridium symbiosum with codeine-paracetamol; Eggerthella lenta and Flavonifractor plautii with clindamycin and dicloxacillin; and Streptococcus parasanguinis and S. salivarius with the proton pump inhibitors pantoprazole and esomeprazole.
This finding matters because stool-based microbiome biomarkers are actively being developed as non-invasive CRC screening tools, and their clinical validity depends on whether detected bacteria reflect tumor biology or simply medication history. The study's 12-year prescription linkage is a methodological strength rarely seen at this scale, enabling separation of current, past, and chronic drug exposure — critical for distinguishing transient from durable microbiome shifts. However, the cohort consisted exclusively of fecal immunochemical test-positive individuals, limiting generalizability to average-risk populations. Causality cannot be established from this observational design; drug-associated bacteria may be epiphenomenal rather than mechanistically linked to neoplasia. The overlap between drug-induced microbial changes and CRC-associated signatures also raises the unsettling possibility that common medications could confound early cancer detection tests, generating false positives or masking true signals. This is a preprint posted on medRxiv and has not yet been peer-reviewed — findings require independent replication before influencing clinical biomarker validation frameworks. Incrementally important, but with immediate relevance to microbiome diagnostics development.