Nursing care
Why does metformin cause lactic acidosis? Liver lactate, kidneys and contrast
Written and reviewed by Nicole Ramirez, BSN, RN, CMSRN · 4 min read · Updated November 2026
Short answer
Metformin can cause lactic acidosis because it reduces the liver's uptake of lactate, so lactate clears more slowly from the blood. The kidneys remove metformin, so when kidney function falls the drug accumulates and lactate climbs further. Low oxygen states, heavy alcohol use, liver impairment, surgery and iodinated contrast all raise the risk.
The liver stops clearing lactate as fast
Everyone makes lactate all day, from muscle, red blood cells and gut. The liver is the main site that takes it back up and recycles it. The metformin label states that the drug decreases liver uptake of lactate, which raises blood lactate levels. In most people that effect is small and the liver keeps up.
MSD Manuals classifies metformin as a cause of type B lactic acidosis, the kind that happens without poor tissue perfusion. That separates it from type A, where shock starves tissues of oxygen and they overproduce lactate. With metformin the main problem is slower removal, not a surge in production, unless a second illness adds a type A problem on top.
Why the kidneys decide how much metformin is on board
Metformin is excreted unchanged by the kidneys, largely through tubular secretion, according to the label. When kidney function drops, its half-life lengthens and blood levels rise. More drug means a stronger brake on liver lactate uptake, and the acid load builds faster than the body can buffer it.
That is the reason for the eGFR rules in the label. Metformin is contraindicated when eGFR is below 30 mL/min/1.73 m2, and starting it is not recommended between 30 and 45. Kidney function is checked before starting and periodically afterward. The numbers make sense once you see that the kidney is the drug's only real exit.
Contrast, surgery and low oxygen: the add-on triggers
Iodinated contrast can cause an acute drop in kidney function. The label describes lactic acidosis after contrast in metformin users, so it advises stopping metformin at the time of or before an iodinated contrast study in clients with an eGFR of 30 to 60, with liver disease, alcoholism or heart failure, or receiving intra-arterial contrast, then rechecking eGFR 48 hours later before restarting.
Surgery with restricted food and fluid is another hold point, because dehydration and low blood pressure threaten the kidneys. Hypoxic states such as acute heart failure, shock or sepsis add type A lactate on top of slower clearance. Heavy alcohol use and liver impairment weaken the liver's ability to clear lactate. Carbonic anhydrase inhibitors such as topiramate also raise the risk.
Early signs are vague, which is how it gets missed
The label lists malaise, muscle aches, breathing difficulty, sleepiness and abdominal pain as early features, with low blood pressure and slow heart rhythms that resist treatment in severe cases. None of these shouts acidosis. A client on metformin who feels generally unwell with abdominal pain after a stomach bug may look like simple gastroenteritis.
Lab work is what confirms it. The label describes lactate above 5 mmol/L with an anion gap acidosis and no ketones in the urine. MSD Manuals uses a blood pH below 7.35 with high lactate as the definition. The nurse holds metformin, reports promptly, and prepares for the treatment the label recommends: stopping the drug and hemodialysis, which removes metformin.
Hypothetical practice scenario
Hypothetical scenario, not a real exam item: a client with type 2 diabetes on metformin is scheduled for a CT with IV contrast tomorrow. The eGFR is 42. Which nursing action is best: give metformin as usual, hold metformin and confirm the plan for rechecking kidney function about 48 hours after the scan, or stop metformin permanently?
Holding it and confirming the recheck is correct, since an eGFR between 30 and 60 puts this client in the group the label says to stop before contrast. Giving it as usual ignores the risk of acute kidney injury from contrast. Stopping it permanently overshoots; the label allows restarting once kidney function is stable.
Sources and further reading
DailyMed: Metformin hydrochloride extended-release tablets label. Decreased liver lactate uptake; renal excretion by tubular secretion and accumulation in renal impairment; eGFR cutoffs; contrast and surgery holds with 48-hour recheck; risk factors; early symptoms; lactate above 5 mmol/L with anion gap and no ketonuria; hemodialysis.
MSD Manual Professional Version: Lactic Acidosis. Definition with pH below 7.35 and high lactate; type A vs type B; biguanides such as metformin as a type B cause.
The next step on this is the same as on everything else here: answer questions and read the rationales. Our pharmacology practice questions are the closest set to what this page covers.
One question from the pharmacology set
A client with heart failure is started on furosemide 40 mg PO daily. Which findings should the nurse report to the provider before administering the next dose? Select all that apply.
Rationale
Furosemide is a loop diuretic, so the two things you are watching are potassium and kidney function. A potassium of 2.9 mEq/L is below the 3.5–5.0 reference range and puts the client at risk for dysrhythmia — hold and report. Muscle cramps with palpitations are the clinical face of that same hypokalemia, so they are reported together, not separately. A creatinine that doubles signals the diuresis has outrun renal perfusion. A blood pressure of 132/78 and a 1 kg loss are the expected response to the drug working, not reasons to hold it.
Answer: A, D, E
Common questions
Why is metformin held before contrast dye?
Iodinated contrast can cause a sudden drop in kidney function. Because metformin leaves the body through the kidneys, it can build up and raise the risk of lactic acidosis. The label advises holding it for at-risk clients and rechecking kidney function about 48 hours after the study before restarting.
What kidney function stops metformin?
The label lists metformin as contraindicated when eGFR is below 30 mL/min/1.73 m2. Starting it is not recommended between 30 and 45. For clients already taking it whose eGFR falls below 45, the prescriber reassesses benefit and risk.
What lab results suggest metformin-associated lactic acidosis?
A high lactate, described in the label as above 5 mmol/L, with an anion gap metabolic acidosis, a low blood pH and no ketones in the urine. Absent ketones help separate it from diabetic ketoacidosis, which also causes an anion gap acidosis.
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