Dr. G:  "what are the proofs that lactic acidosis ... may be exacerbated by this action  (using metformin concurrently with radiocontrast image)?"   I don't know the specific studies it was based on but it is common enough that there is now a black box warning on metformin about this; and LAST WEEK, in the hospital where I work, they killed a patient in exactly this way.  He was on metformin, nobody discontinued it, he had a CT scan with contrast, and within 3 days he developed increasing lactic acidosis and died of it.  I have been researching this and apparently this is the general chain of events:  dyes impair kidney function, at best temporarily but I have seen several patients whose kidneys were permanently damaged by contrast medium. The patient is supposed to be instructed to drink copious amounts of water before and after the procedure to dilute the concentration of dye as it passes through - but often this critical step is neglected.  Lactic acidosis develops when lactate, a product of carbohydrate metabolism, accumulates faster than the liver can metabolize it.  Metformin is eliminated through the kidneys.  When this filtration is impaired the blood levels of it increase greatly.  Too much carbohydrate metabolism goes on and the lactate builds up, the blood pH goes down, you have acidosis.