Metformin has been shown to significantly increase 18F-FDG uptake in the colon and, to a lesser extent, the small bowel.
Cells of the intestinal wall are exposed to much higher concentrations of metformin for much longer times compared to other cell types. In addition, animal studies have shown that metformin increases glucose transfer into intestinal mucosal cells and can increase glucose utilization by up to 60%.
This results in intense, diffuse, and continuous uptake along the bowel on 18F-FDG PET imaging. The uptake is seen in both the wall and within the lumen (likely due to excretion of 18F-FDG into the stool).
The pattern is fairly characteristic and confusion with malignant focal bowel uptake is rare. The problem is that this diffuse uptake can mask an existing bowel malignancy and lead to a false-negative result.