Fluorodeoxyglucose is a glucose analog. Its full chemical name is 2-fluoro-2-deoxy-D-glucose, commonly abbreviated to FDG.

FDG is most commonly used in the medical imaging modality positron emission tomography (PET): the fluorine in the FDG molecule is chosen to be the positron-emitting radioactive isotope fluorine-18, to produce 18F-FDG. After FDG is injected into a patient, a PET scanner can form images of the distribution of FDG around the body. The images can be assessed by a nuclear medicine physician or radiologist to provide diagnoses of various medical conditions.

FDG, as a glucose analog, is taken up by high-glucose-using cells such as brain, kidney, and cancer cells, where phosphorylation prevents the glucose from being released intact. The 2-oxygen in glucose is needed for further glycolysis, so that (in common with 2-deoxy-D-glucose) FDG cannot be further metabolized in cells, and therefore the FDG-6-phosphate formed does not undergo glycolysis before radioactive decay. As a result, the distribution of 18F-FDG is a good reflection of the distribution of glucose uptake and phosphorylation by cells in the body.