Contraindications to Metformin Use
Metformin is absolutely contraindicated in severe renal impairment (eGFR <30 mL/min/1.73 m²), decompensated cirrhosis (Child-Pugh class B-C), and acute metabolic acidosis including diabetic ketoacidosis. 1, 2
Absolute Contraindications
Severe Renal Impairment
- eGFR <30 mL/min/1.73 m² is an absolute contraindication due to metformin accumulation and increased risk of lactic acidosis 1, 3
- Metformin is renally excreted, and clearance is proportional to GFR, making accumulation inevitable below this threshold 4
- The FDA explicitly contraindicates use when eGFR falls below 30 mL/min/1.73 m² 1
Decompensated Liver Disease
- Metformin is contraindicated in decompensated cirrhosis (Child-Pugh class B-C), especially with concurrent renal dysfunction 2
- Hepatic impairment increases lactic acidosis risk due to impaired oxidative phosphorylation and reduced lactate clearance 2
- The FDA states metformin is not recommended in patients with hepatic impairment due to lactic acidosis cases 1
Metabolic Acidosis
- Acute or chronic metabolic acidosis, including diabetic ketoacidosis (with or without coma), is an absolute contraindication 1
Hypersensitivity
- Known hypersensitivity to metformin contraindicates its use 1
Situations Requiring Immediate Discontinuation
Acute Illness and Hospitalization
- Discontinue metformin during any hospitalization, acute illness, or condition that may compromise renal or hepatic function 5, 3, 2
- Stop metformin during serious infections, sepsis, dehydration, vomiting, diarrhea, acute heart failure with hypoperfusion/hypoxemia, and intestinal obstruction 2
- Implement "sick-day rules" where patients are educated to hold metformin when acutely ill at home 4, 3
Procedures and Contrast Media
- Discontinue metformin before procedures involving iodinated contrast media in patients with eGFR 30-60 mL/min/1.73 m² or those with liver disease, alcoholism, or heart failure 3
- Hold metformin around surgery and angiography to prevent acute kidney injury 4
Relative Contraindications and Cautions
Moderate Renal Impairment (eGFR 30-44 mL/min/1.73 m²)
- Do not initiate metformin when eGFR is 30-44 mL/min/1.73 m² 6
- If already on metformin, reduce dose to maximum 1000 mg/day and monitor renal function every 3-6 months 3, 6
- The FDA recommends assessing benefits and risks of continuing treatment when eGFR falls below 45 mL/min/1.73 m² 7
Compensated Cirrhosis (Child-Pugh Class A)
- Metformin can be used cautiously in compensated cirrhosis only if eGFR ≥45 mL/min/1.73 m² 2
- Requires monitoring of renal function every 3-6 months due to increased risk of worsening kidney function 2
Congestive Heart Failure
- Use metformin with caution in patients with congestive heart failure due to increased lactic acidosis risk 5, 1
- This is particularly relevant when heart failure is associated with hypoperfusion or hypoxemia 2
Respiratory Insufficiency
- Metformin accumulation may increase lactic acidosis risk if concomitant respiratory insufficiency is present 8
- Conditions causing tissue hypoxia increase lactate production, making metformin use hazardous 8
Clinical Pitfalls to Avoid
Over-reliance on Serum Creatinine Alone
- Do not use serum creatinine alone to assess metformin eligibility—always calculate eGFR using validated formulas (MDRD or CKD-EPI) 9, 10
- Using creatinine cutoffs of 1.4-1.5 mg/dL would unnecessarily exclude 12.4% of patients who have eGFR >30 mL/min/1.73 m² 10
- Creatinine-based criteria disproportionately affect elderly patients and women, leading to inappropriate metformin withholding 9
Elderly Patients
- Assess renal function more frequently in elderly patients due to age-related decline (approximately 40% reduction by age 70) 6, 1
- Start at the low end of dosing range and monitor for gastrointestinal side effects and vitamin B12 deficiency 1, 8
- Set relaxed glycemic targets (A1C 7.5-8.5%) in very elderly patients to minimize hypoglycemia risk 6