Fluorine is critical for biomedicine. This element can help drug compounds be more potent and last longer in the body, and its radioactive isotope, fluorine-18, powers medical imaging techniques such as positron emission tomography (PET). But scientists have long struggled with adding fluorine to the most common chemical bonds—carbon–hydrogen (C–H) bonds—in a way that's precise, efficient and compatible with the molecules used to create many modern medicines.