Iron deficiency and anaemia are common in colorectal cancer. Replacement with oral or intravenous iron effectively treats this deficiency. However, mechanistic and population studies suggest that excess iron promotes colorectal carcinogenesis. Growing research into gut microbiota and dysbiosis suggests one explanation for this association. Iron is growth limiting for many pathogenic bacteria and may promote a shift in the ratio of pathogenic to protective bacteria. This may increase the toxic bacterial metabolites, promoting inflammation and carcinogenesis. This has important implications as we seek to correct anaemia in our patients.