Definition: Lactic acidosis is a build-up of lactic acid in the blood. It is the boxed warning on metformin labels, which say cases reported after approval have caused death. It is rare and often starts with vague symptoms such as feeling unwell, muscle aches, trouble breathing, sleepiness and stomach pain.
The metformin label lists:
In reported cases, blood lactate was above 5 mmol/L. The Patient Information lists signs such as feeling very weak and tired, unusual muscle pain, trouble breathing, unusual sleepiness, feeling cold in the arms and legs, dizziness, or a slow or irregular heartbeat, and says to stop metformin and call the provider right away.
The label ties metformin use to eGFR. It is contraindicated below 30 mL/min/1.73 m2 and not started between 30 and 45. Kidney function is checked before starting and at least once a year. The label also pauses metformin around some contrast scans and when food and fluids are restricted for surgery.
The pause rules depend on dates: a scan, a procedure, a hospital stay. A log of when you paused and restarted, next to your eGFR results, helps every clinician see the timeline. This is a different acid problem from diabetic ketoacidosis, which metformin is contraindicated in.
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