Metformin
High-yield Verified · Oct 2026Metformin tells the liver to make less glucose and leaves the β cell alone — which is why it rarely causes hypoglycemia by itself. Most people with type 2 diabetes start with it. Its boxed warning, lactic acidosis, is what happens when drug accumulation (the kidneys clear it unchanged) meets extra lactate or poor lactate clearance (hypoxia, alcohol, liver disease).
Nursing view → administration, monitoring and patient teaching
Compare: Diabetes drugs compared — where they act, hypoglycemia, weight and outcomes →
Drugs
Mechanism
Metformin, a biguanide, decreases hepatic glucose production, decreases intestinal absorption of glucose and improves insulin sensitivity by increasing peripheral glucose uptake and utilization; it does not force insulin out, so on its own it is unlikely to cause hypoglycemia.
Less glucose from the liver, no extra insulin
Metformin is a biguanide. It lowers both basal and postprandial glucose by decreasing hepatic glucose production, decreasing intestinal absorption of glucose, and improving insulin sensitivity by increasing peripheral glucose uptake and use. Insulin secretion remains unchanged; fasting insulin and the day-long insulin response may even fall.
Because it does not force insulin out, metformin is unlikely to cause hypoglycemia on its own and is weight-neutral, with the potential for modest weight loss. The hypoglycemia risk rises when it is combined with insulin or an insulin secretagogue such as a sulfonylurea (how secretagogues release insulin is on the Sulfonylureas page). Most people with type 2 diabetes start with metformin.
Lactate: who makes it, who clears it
Lactate is produced every day — by skin, red cells, brain, muscle and the gut — and is metabolized mainly by the liver and kidneys, the only organs with the enzymes for gluconeogenesis from lactate. Lactic acidosis occurs when production exceeds clearance, and it is most severe when excess production and impaired hepatic clearance coexist.
Lactic acidosis is classed as type A (tissue hypoperfusion and hypoxia shift metabolism to anaerobic glycolysis) or type B (no hypoxia or hypoperfusion). Biguanide therapy is a cause of type B; both types share the mitochondria’s inability to metabolize excess pyruvate. Metformin’s predecessor, phenformin, was withdrawn for a high incidence of fatal lactic acidosis.
What toxic metformin does to the mitochondria
Supratherapeutic metformin inhibits complex I of the mitochondrial respiratory chain, impairing oxidative phosphorylation, and inhibits pyruvate carboxylase, so more pyruvate is converted to lactate. As ATP generation falls, AMP rises and inhibits fructose-1,6-bisphosphatase, the rate-limiting enzyme of gluconeogenesis — and impaired gluconeogenesis reduces the liver’s clearance of lactate. Inhibition of glucose-6-phosphatase also impairs glycogenolysis; reduced gluconeogenesis and glycogenolysis together contribute to the hypoglycemia occasionally seen in metformin-associated lactic acidosis.
The acidosis itself comes from the hydrogen ions. With complex I blocked, mitochondria cannot recycle and buffer the hydrogen ions produced by ATP hydrolysis; they accumulate and overcome the body’s buffers. The lactate is a marker of anaerobic metabolism rather than the direct cause of the acidosis. Renal and hepatic failure make it worse by reducing metformin and lactate clearance, respectively.
The boxed warning: accumulation meets extra lactate
Metformin is excreted unchanged in the urine, with no significant hepatic metabolism or biliary excretion. Its renal clearance is about 3.5 times the creatinine clearance, so tubular secretion is the main route; renal excretion is mediated by organic cation transporters (OCT) and MATE. The risk of accumulation and lactic acidosis rises with the severity of renal impairment — most postmarketing cases occurred with significant renal impairment — so metformin is contraindicated below an eGFR of 30, and starting it between 30 and 45 is not recommended.
The other risk states each act on one side of the balance. Iodinated contrast has caused an acute fall in renal function; withheld food and fluids around procedures cause volume depletion, hypotension and renal impairment. Hypoxic states — acute heart failure with hypoperfusion, shock, myocardial infarction, sepsis — add lactate and can cause prerenal azotemia. Hepatic impairment reduces lactate clearance, and alcohol potentiates metformin’s effect on lactate metabolism. Older adults are more likely to have hepatic, renal or cardiac impairment. Carbonic anhydrase inhibitors lower serum bicarbonate, and drugs that block OCT2/MATE raise metformin exposure. Mitochondrial diseases such as MELAS and MIDD also raise the risk.
The gut and vitamin B12
GI effects — diarrhea, nausea and vomiting — affect up to 30% of patients and are the commonest adverse reactions; diarrhea led to discontinuation in 6% of patients in trials. Giving metformin with a meal reduces GI upset, and the dose is titrated up gradually to limit it.
Metformin can lower vitamin B12: in 29-week trials, about 7% of patients with previously normal levels developed subnormal ones, possibly through interference with B12 absorption from the B12–intrinsic factor complex. The fall may be associated with anemia but appears rapidly reversible when metformin is stopped or B12 is supplemented; people with inadequate B12 or calcium intake or absorption are predisposed. Hematologic parameters are measured yearly and B12 every 2 to 3 years.
Comparing the drugs
Lactic-acidosis risk states on the metformin label
| Drug | Why it raises the risk | What the label says to do |
|---|---|---|
| eGFR below 30 | Metformin is substantially excreted by the kidney, so it accumulates | Contraindicated |
| eGFR 30 to 45 | Less renal clearance | Do not start; if eGFR falls below 45 on therapy, weigh benefit and risk |
| Iodinated contrast | Has caused an acute decrease in renal function | Stop at or before the study when eGFR is 30–60, with hepatic impairment, alcoholism or heart failure, or for intra-arterial contrast; recheck eGFR at 48 hours |
| Surgery and other procedures | Withheld food and fluids → volume depletion, hypotension, renal impairment | Stop temporarily while intake is restricted |
| Hypoxic states (acute heart failure, shock, MI, sepsis) | Hypoperfusion and hypoxemia; can cause prerenal azotemia | Discontinue |
| Hepatic impairment | Impaired lactate clearance | Avoid |
| Excess alcohol | Potentiates metformin’s effect on lactate metabolism | Warn against excessive intake |
| Carbonic anhydrase inhibitors (e.g., topiramate) | Lower serum bicarbonate; non–anion gap acidosis | Monitor more often |
| OCT2/MATE inhibitors (e.g., ranolazine, dolutegravir, cimetidine) | Reduce renal tubular elimination, raising metformin exposure | Weigh benefits and risks |
| Mitochondrial disease (MELAS, MIDD) | Higher risk of lactic acidosis | Not recommended |
Pharmacokinetics
- Absorption
- Absolute bioavailability is about 50–60% under fasting conditions, and absorption is less than dose-proportional at higher doses. Food decreases and slightly delays absorption. Intestinal absorption is mediated by organic cation transporters.
- Distribution
- Negligible plasma protein binding and a large apparent volume of distribution; metformin partitions into erythrocytes, and much of the drug resides in tissues such as red cells, intestine, skeletal muscle and liver.
- Metabolism
- None of significance — metformin is excreted unchanged, with no significant hepatic metabolism or biliary excretion.
- Elimination
- Renal, mainly by tubular secretion (renal clearance about 3.5 times creatinine clearance): about 90% of the absorbed drug leaves in the urine within 24 hours. The plasma half-life is about 6.2 hours, and about 17.6 hours in blood, reflecting the erythrocyte compartment. With reduced renal function the half-life lengthens and renal clearance falls; the changes with age mainly reflect renal function.
Uses & drug choice
- Type 2 diabetes — with diet and exercise to improve glycemic control (adults and children 10 years and older); most people with type 2 diabetes start with metformin
Adverse effects
- Diarrhea, nausea, vomiting
- The commonest effects, in up to 30% of patients; giving metformin with a meal and increasing the dose gradually reduce them.
- Vitamin B12 deficiency
- Possibly from interference with B12 absorption from the B12–intrinsic factor complex; may be associated with anemia and reverses rapidly when the drug is stopped or B12 is given.
- Lactic acidosis
- Accumulation (renal impairment) plus extra lactate (hypoxia) or impaired clearance (liver disease, alcohol); at toxic levels metformin blocks mitochondrial complex I and gluconeogenesis, so lactate builds up and hydrogen ions are not recycled.
- Hypoglycemia, only in combination
- Metformin leaves insulin secretion unchanged, so hypoglycemia comes from the insulin or insulin secretagogue it is combined with; a lower dose of that drug may be needed.
Toxicity & antidote
Metformin-associated lactic acidosis is defined as a pH below 7.35 with lactate above 5 mmol/L in the setting of metformin use or overdose, and its onset is often subtle: malaise, myalgias, respiratory distress, somnolence and abdominal pain. Postmarketing cases have caused death, hypothermia, hypotension and resistant bradyarrhythmias. Lactic acidosis was reported in about 32% of overdose cases, and hypoglycemia in about 10% (no causal link established).
There is no antidote. If lactic acidosis is suspected, metformin is stopped immediately and supportive care is started in hospital, with prompt hemodialysis: metformin is easily dialyzable because of its small molecular weight and minimal protein binding, and dialysis also corrects the acidosis.
Interactions
Contraindications
- eGFR < 30 mL/min/1.73 m², or acute/chronic metabolic acidosis (including DKA) — Metformin is substantially excreted by the kidney, so impaired clearance lets it accumulate; it also decreases the liver’s uptake of lactate, raising blood lactate, which may increase the risk of lactic acidosis in patients at risk.
- Around iodinated contrast (hold and recheck renal function) use caution — With eGFR 30–60, liver disease, alcoholism, heart failure, or intra-arterial contrast, metformin is held at the time of the study and restarted only after eGFR is re-evaluated at 48 hours and stable.
High-yield
Practice questions
Q1A 71-year-old woman with type 2 diabetes on metformin is admitted with urosepsis and acute kidney injury. Her pH is 7.08, lactate 11 mmol/L, and the anion gap is high.
Which property of metformin explains why it accumulated?
- A.It is extensively metabolized by hepatic CYP enzymes
- B.It is highly protein bound and displaced by acidosis
- C.It is excreted unchanged by the kidneys, mainly by tubular secretion
- D.It is cleared mainly by biliary excretion
- E.It is converted to an active metabolite cleared by the kidneys
Show answer
C. It is excreted unchanged by the kidneys, mainly by tubular secretion — Metformin is excreted unchanged in the urine: renal clearance is about 3.5 times creatinine clearance, so tubular secretion is the main route, and the risk of accumulation and lactic acidosis rises with the severity of renal impairment. It undergoes no significant hepatic metabolism or biliary excretion, so it has no active metabolite, and it is negligibly bound to plasma proteins. Sepsis adds lactate and can cause prerenal azotemia, so metformin is discontinued.
Q2A 52-year-old man started metformin monotherapy for type 2 diabetes three months ago. He fasts from sunrise to sunset for a month and is surprised that his glucose never falls below 80 mg/dL.
Why is hypoglycemia unlikely on metformin alone?
- A.It lowers hepatic glucose output without increasing insulin secretion
- B.It closes β-cell K-ATP channels only when glucose is high
- C.It blocks renal glucose reabsorption, which stops at low glucose
- D.It works through an active metabolite formed only after meals
Show answer
A. It lowers hepatic glucose output without increasing insulin secretion — Metformin decreases hepatic glucose production, decreases intestinal glucose absorption and improves peripheral glucose uptake, while insulin secretion remains unchanged — so it is unlikely to cause hypoglycemia unless it is combined with insulin or an insulin secretagogue such as a sulfonylurea. Because insulin secretion is unchanged, it does not work by releasing insulin from the β cell; its three actions are on hepatic glucose production, intestinal glucose absorption and peripheral glucose uptake, none of them in the kidney; and it is excreted unchanged, with no significant hepatic metabolism, so it has no active metabolite.
Q3A 64-year-old man with type 2 diabetes on metformin has an eGFR of 48 mL/min/1.73 m². He is scheduled for a CT angiogram with IV iodinated contrast tomorrow.
What should happen to his metformin?
- A.Continue it; iodinated contrast does not affect metformin clearance
- B.Stop it permanently and switch to another agent
- C.Stop it only if the eGFR falls below 30 after the scan
- D.Stop it at or before the scan, recheck eGFR 48 hours later, and restart if renal function is stable
Show answer
D. Stop it at or before the scan, recheck eGFR 48 hours later, and restart if renal function is stable — Intravascular iodinated contrast has caused an acute decrease in renal function and lactic acidosis in metformin-treated patients. With an eGFR between 30 and 60, the label says to stop metformin at the time of, or before, the study, re-evaluate eGFR 48 hours afterwards and restart if renal function is stable — so it is neither continued through the scan, nor stopped for good, nor held only after the eGFR has already fallen below 30.
Q4A 34-year-old woman is brought in 5 hours after swallowing a bottle of her mother’s metformin. She is tachypneic, with pH 6.95 and lactate 14 mmol/L.
Which action of metformin at toxic levels most directly produces her acidemia?
- A.Stimulation of hepatic gluconeogenesis from lactate
- B.Inhibition of mitochondrial complex I, so hydrogen ions from ATP hydrolysis are not recycled
- C.Renal bicarbonate wasting like that of carbonic anhydrase inhibitors
- D.Lactate itself, as the direct acid load
Show answer
B. Inhibition of mitochondrial complex I, so hydrogen ions from ATP hydrolysis are not recycled — Supratherapeutic metformin inhibits complex I of the mitochondrial respiratory chain; mitochondria can then no longer recycle and buffer the hydrogen ions produced by ATP hydrolysis, and these accumulate to produce the metabolic acidosis. Hyperlactatemia is a marker of anaerobic metabolism rather than the direct cause. Metformin impairs, rather than stimulates, gluconeogenesis — reducing hepatic lactate clearance — and the acidosis is an anion-gap acidosis, unlike the non–anion gap acidosis of carbonic anhydrase inhibitors. Treatment is supportive care with prompt hemodialysis.
Sources
- Metformin hydrochloride tablets (Zydus) — mechanism, pharmacokinetics, lactic acidosis boxed warning, B12, interactions (FDA label) — FDA / DailyMed
- Metformin — mechanism, weight neutrality, adverse effects, toxicity — StatPearls (NCBI)
- Metformin-Associated Lactic Acidosis (MALA) — complex I, gluconeogenesis, hydrogen ions, dialysis — StatPearls (NCBI)
- Lactic Acidosis — lactate production and clearance, type A and type B — StatPearls (NCBI)
- Insulin, Medicines, & Other Diabetes Treatments — most people with type 2 diabetes start with metformin — NIDDK (National Institutes of Health)
By LearnDrugs
Educational summary for medical students. Always verify against current prescribing information and guidelines. Not medical advice.