What is the recommended dose of metformin for an adult patient with type 2 diabetes and normal renal function?

Metformin Dosing in Type 2 Diabetes

Standard Dosing for Normal Renal Function

For adults with type 2 diabetes and normal renal function (eGFR ≥60 mL/min/1.73m²), start metformin at 500 mg twice daily or 850 mg once daily with meals, and titrate up to a maximum of 2550 mg per day in divided doses based on glycemic control and tolerability. 1

Initial Dosing Strategy

• Begin with 500 mg orally twice daily or 850 mg once daily, administered with meals to minimize gastrointestinal side effects 1
• Increase the dose in increments of 500 mg weekly or 850 mg every 2 weeks based on glycemic control and tolerability 1
• The maximum recommended dose is 2550 mg per day, though doses above 2000 mg may be better tolerated when given three times daily with meals 1

Dose Adjustments Based on Renal Function

For patients with eGFR 45-59 mL/min/1.73m²:

• Continue the current metformin dose without increase 2
• Consider dose reduction in elderly patients or those with concomitant liver disease 2
• Monitor renal function every 3-6 months 2, 3

For patients with eGFR 30-44 mL/min/1.73m²:

• Reduce metformin to half the maximum recommended dose (maximum 1000 mg total daily dose, typically 500 mg twice daily) 2, 1
• Initiation of metformin is not recommended in this eGFR range 1
• Monitor renal function every 3-6 months 2

For patients with eGFR <30 mL/min/1.73m²:

• Metformin is absolutely contraindicated and must be discontinued immediately due to risk of lactic acidosis 4, 2, 1

Important Monitoring Requirements

• Assess renal function prior to initiation and periodically thereafter, at least every 3-6 months when eGFR is <60 mL/min/1.73m² 2, 3, 1
• Monitor for vitamin B12 deficiency in patients on long-term metformin therapy (>4 years) 2, 3
• Discontinue metformin at the time of or prior to iodinated contrast imaging procedures in patients with eGFR 30-60 mL/min/1.73m², and re-evaluate eGFR 48 hours after the procedure before restarting 1

Common Pitfalls to Avoid

• Do not continue metformin at any dose when eGFR falls below 30 mL/min/1.73m² - this is a hard contraindication regardless of prior tolerance 4, 1
• Do not initiate metformin in patients with eGFR 30-45 mL/min/1.73m² 1
• Implement "sick day rules" to temporarily stop metformin during serious intercurrent illness that increases risk of acute kidney injury 2

Formulation Considerations

• Extended-release metformin formulations may improve gastrointestinal tolerability compared to immediate-release formulations, with comparable glycemic control at the same total daily dose 5, 6
• Extended-release formulations permit once-daily dosing, which may enhance patient compliance 7, 6