Yes, hold metformin the night before shoulder surgery
Stop metformin the night before the surgery (approximately 24-36 hours preoperatively) and do not restart it until at least 48 hours postoperatively after confirming adequate renal function. 1
Rationale for Holding Metformin
The primary concern is metformin-associated lactic acidosis (MALA), a rare but serious complication with a mortality rate of 30-50%. While the absolute incidence is low (2-9 cases per 100,000 patients/year), the perioperative period creates multiple risk factors that substantially increase this risk 1.
Key Risk Factors Present During Surgery:
- Perioperative fasting and fluid restriction leading to dehydration
- Potential hemodynamic instability during anesthesia
- Tissue hypoperfusion from surgical stress
- Acute kidney injury risk from volume depletion or hypotension
- Altered renal function from the surgical stress response
The FDA label explicitly states that metformin should be withheld during "restricted food and fluid intake" and when conditions may cause "volume depletion, hypotension, and renal impairment" 2.
Specific Pre-Screening Required
Before surgery, assess these critical risk factors that would make MALA more likely 1:
- Renal function: Check eGFR (must be >30 mL/min; ideally >60 mL/min)
- Cardiac status: Severe heart failure (LVEF <30%) increases risk
- Concurrent medications: ACE inhibitors, ARBs, diuretics, NSAIDs all impair renal function
- Age: Patients >65 years have higher risk
- Metformin dose: High doses (>2000 mg/day) associated with more cases
Timing Protocol
Preoperatively:
- Stop metformin the night before surgery (last dose approximately 24-36 hours before procedure) 1
Postoperatively:
- Do not restart before 48 hours after surgery 1
- Verify adequate renal function (check eGFR) before restarting 1, 2
- Ensure patient is eating/drinking normally and hemodynamically stable
Important Caveats
For minor/ambulatory procedures: Some guidelines suggest metformin can be continued if there is no severe renal failure 1. However, shoulder surgery typically involves general anesthesia, fasting, and potential for hemodynamic changes—making it more appropriate to follow major surgery protocols.
Glycemic control during the hold: The patient will need alternative glucose management during the metformin-free period. Coordinate with anesthesia regarding intraoperative glucose monitoring and potential need for insulin coverage if blood glucose exceeds 180 mg/dL (10 mmol/L) 3.
Lactic acidosis monitoring: If the patient has multiple risk factors (elderly, high metformin dose, borderline renal function), consider checking lactate levels postoperatively before restarting metformin 3, 2.
Evidence Quality Note
While the absolute risk of MALA is low and some recent studies suggest metformin continuation may be safe in select patients 4, 5, the guideline consensus strongly favors discontinuation [1-1]. The 30-50% mortality rate when MALA does occur, combined with the multiple perioperative risk factors present during surgery, makes the conservative approach of holding metformin the safest recommendation. Recent data even shows potential benefits of metformin use in shoulder surgery patients 6, but these studies don't address the acute perioperative period risk—they examine chronic preoperative use patterns.
The risk-benefit analysis clearly favors a 48-hour perioperative hold to prevent a potentially fatal complication.