We have been screening medications for their ability to inhibit the PCR detection of Mtb. PCR inhibitors can cause false negative PCR diagnostic results and corresponding misdiagnoses. It is important that regulatory reviewers, laboratory technicians and physicians be informed about such deficiencies and limitations of diagnostic kits that are based on nucleic acid amplification systems, to aid them in making informed medical and regulatory decisions concerning the use of such diagnostic systems. The PCR test system we are using involves amplification of segments of DNA isolated from an attenuated strain of Mtb (H37Ra). We found that small (micromolar) quantities of "FE-TABS", an iron supplement and over-the-counter medication, readily inhibited PCR. Similar small (micromolar) amounts of ferrous sulfate, the "active" ingredient in FE-TABS, also readily inhibited PCR. We found by comparison on a molar basis that ferrous sulfate was nearly as inhibitory as the well-known PCR inhibitor hematin, and that ferric sulfate, a compound associated with ferrous sulfate in vivo, was equally as PCR inhibitory as hematin. Hematin contains iron; therefore, our experiments suggest that iron-containing medications may interfere with PCR-based diagnostics.