Background: Epidemiological studies indicate that aspirin consumption reduces the risk of tumors, which is especially relevant for colonic adenoma and carcinoma. Similar observations were made for glial brain tumors and breast cancers, yet the results are inconsistent. Gentisic acid (GA) is a minor catabolite of aspirin; yet humans carrying CYP2C9-variants incapable to catabolize aspirin to GA do not benefit from aspirin in prevention against colonic adenoma. GA blocks binding of Fibroblastic Growth Factor to its receptor and its sulphonate metabolite dobesilic acid blocks growth of C6 glioblastoma in vivo. GA is also an endogenously produced siderophore in mammalians for the transport of iron, a trace element which stimulates tumor growth and enhances anthracycline cardiotoxicity.