Why should contrast media not be given to patients taking Metformin?

The reason that contrast media should not be administered to patients taking Metformin stems from the risk of lactic acidosis and contrast-induced nephropathy (CIN). Metformin is primarily eliminated by the kidneys, and when contrast media is used, there is a potential risk for acute kidney injury, especially in patients who may already have compromised renal function.

If renal function is impaired after the administration of contrast media, the elimination of Metformin from the body could be significantly reduced, leading to an accumulation of the drug in the bloodstream. This may raise the risk of lactic acidosis, a serious and potentially life-threatening condition characterized by an accumulation of lactic acid in the body due to anaerobic metabolism.

While allergic reactions to contrast media can occur, and interference with kidney function is a concern, these are not the primary reasons Metformin is withheld. Additionally, hyperglycemia can happen for various reasons, but it is not specifically tied to the use of contrast media in patients on Metformin. Thus, the greatest concern is the combination of potential lactic acidosis and contrast-induced nephropathy.