The prescribing information for metformin ER tablets carries a boxed warning on metformin-associated lactic acidosis. It states that postmarketing cases have resulted in death, hypothermia, hypotension, and resistant bradyarrhythmias. Studies reported different findings in different groups of people, so this guide gives each source's own words and numbers.
What the boxed warning says
The boxed warning says the onset of metformin-associated lactic acidosis is often subtle, accompanied only by nonspecific symptoms such as malaise, myalgias, respiratory distress, somnolence, and abdominal pain. It also gives a description of the condition as characterized by elevated blood lactate levels (> 5 mmol/Liter), anion gap acidosis (without evidence of ketonuria or ketonemia), an increased lactate/pyruvate ratio, and metformin plasma levels generally > 5 mcg/mL.
The boxed warning lists renal impairment, concomitant use of certain drugs (e.g., carbonic anhydrase inhibitors such as topiramate), and age 65 years old or greater among the risk factors. The highlights summary lists renal impairment, concomitant use of certain drugs, age > 65 years old, radiological studies with contrast, surgery and other procedures, hypoxic states, excessive alcohol intake, hepatic impairment, and mitochondrial diseases.
Interactions the label names
The highlights of the label state that carbonic anhydrase inhibitors may increase risk of lactic acidosis and say to consider more frequent monitoring. The drug interactions table gives topiramate, zonisamide, acetazolamide or dichlorphenamide as examples.
The highlights also state that drugs that reduce metformin clearance (such as ranolazine, vandetanib, dolutegravir, and cimetidine) may increase the accumulation of metformin, and say to consider the benefits and risks of concomitant use. On alcohol, they state that alcohol can potentiate the effect of metformin on lactate metabolism, and say to warn patients against excessive alcohol intake.
What studies found
These three sources differ in design and in the people studied.
| Question | What the source says |
|---|---|
| What did the 2010 Cochrane systematic review report in people with type 2 diabetes? | The Cochrane review abstract reports that pooled data from 347 comparative trials and cohort studies revealed no cases of fatal or nonfatal lactic acidosis in 70,490 patient-years of metformin use or in 55,451 patient-years in the non-metformin group. It concluded that there is no evidence from prospective comparative trials or from observational cohort studies that metformin is associated with an increased risk of lactic acidosis, or with increased levels of lactate, compared to other anti-hyperglycemic treatments. |
| What did an observational community-based cohort study of hospitalization with acidosis find? | The cohort study reported that metformin use was associated with an increased risk of acidosis at eGFR less than 30 mL/min/1.73 m2 (adjusted HR, 2.07; 95% CI, 1.33-3.22). Its conflict of interest statement says one author serves on the Advisory Board of MesaRx Innovations and holds equity in Monument Analytics, a health care consultancy whose clients include the life sciences industry. |
| What did a 2026 meta-analysis report in people with chronic kidney disease and type 2 diabetes? | The systematic review and meta-analysis of observational studies concluded that, based on observational evidence with very low certainty, metformin was inversely associated with all-cause mortality and ESRD. However, the results were inconsistent, and a potential increase in lactic acidosis risk in stage 4 was observed. |
Symptoms and what the sources say to do
The warnings section says to educate patients and their families about the symptoms of lactic acidosis and, if these symptoms occur, to instruct them to discontinue the tablets and report these symptoms to their healthcare provider. The boxed warning says that if lactic acidosis is suspected, immediately discontinue the tablets and institute general supportive measures in a hospital setting, and that prompt hemodialysis is recommended.
The patient information leaflet says to stop taking the tablets and call your healthcare provider right away if you get any of the following symptoms of lactic acidosis: feel very weak and tired, have unusual (not normal) muscle pain, have trouble breathing. The list continues with unusual sleepiness or sleeping longer than usual; unexplained stomach or intestinal problems with nausea and vomiting, or diarrhea; feeling cold, especially in your arms and legs; feeling dizzy or lightheaded; and a slow or irregular heartbeat. The leaflet says lactic acidosis is a medical emergency and must be treated in a hospital.
For an overdose, the label's overdosage section says to consider contacting the Poison Help line (1-800-222-1222) or a medical toxicologist for additional overdosage management recommendations. The patient information says that if you take too much, call your healthcare provider, local Poison Control Center, or go to the nearest hospital emergency room right away.
What this does not tell you
The three studies differ in design and population, and their results do not combine into one number for any person. The cohort study was observational and the meta-analysis pooled observational studies, and observational studies cannot show cause and effect. This guide does not call the risk low or high. It gives the sources' words and numbers. The label's risk factor lists and the patient information are statements in those documents. They do not say whether any risk factor applies to a particular person, and a licensed provider decides.