The reliable supply of molybdenum-99 (99Mo) and its decay product, technetium-99m (99mTc), is a vital component of modern medical diagnostic practices. At present, most of the global production of 99Mo is from highly enriched uranium (HEU) targets. However, all major 99Mo-producing countries have recently agreed to convert to using low-enriched uranium (LEU) targets to advance important non-proliferation goals, a decision that will have implications for the global supply chain of 99Mo/99mTc and the long-term supply reliability of these medical isotopes. This study provides the findings and analysis from an extensive examination of the 99Mo/99mTc supply chain by the OECD/NEA High-level Group on the Security of Supply of Medical Radioisotopes (HLG-MR). It presents a comprehensive evaluation of the potential impacts of converting to the use of LEU targets for 99Mo production on the global 99Mo/99mTc market in terms of costs and available production capacity, and the corresponding implications for long-term supply reliability. In this context, the study also briefly discusses the need for policy action by governments in their efforts to ensure a stable and secure long-term supply of 99Mo/99mTc.