Should metformin be withheld in patients who are nothing by mouth (NPO)?

Metformin Should Be Withheld in NPO Patients

Yes, metformin should be temporarily discontinued in patients who are NPO, particularly when NPO status is associated with surgery, procedures requiring contrast, acute illness, or risk of dehydration and acute kidney injury.

Guideline-Based Recommendations

The Canadian Society of Nephrology explicitly recommends applying "sick-day rules" for metformin management 1:

• Metformin should be temporarily discontinued when patients face serious intercurrent illness that increases AKI risk, including situations around surgery and angiography 1
• This recommendation specifically applies to patients with GFR < 60 mL/min/1.73 m², but the principle extends to all patients in high-risk situations 1
• Patients should not take metformin on days when they may experience AKI, including around surgery 1

The FDA drug label reinforces this approach 2:

• Metformin must be stopped when patients are "going to have surgery and not able to eat or drink much" 2
• The medication should be restarted only after discussing timing with the healthcare provider 2

Clinical Reasoning: Why NPO Status Matters

Risk of Lactic Acidosis

The primary concern is metformin-associated lactic acidosis (MALA), which occurs when 2:

• Dehydration develops (lose large amount of body fluids from being sick, vomiting, diarrhea, or inadequate fluid intake) 2
• Renal function deteriorates (metformin accumulates when kidneys cannot clear it properly) 2
• Tissue hypoperfusion occurs (surgery, acute illness, hemodynamic instability) 2

NPO status creates a perfect storm for these risk factors 3:

• Patients cannot maintain oral hydration 3
• Volume depletion can precipitate acute kidney injury 3
• Metformin accumulation accelerates lactate production 3

Procedure-Specific Considerations

For surgical procedures, the European Society of Cardiology guidelines note 1:

• Metformin should be suspended before procedures, though they acknowledge "no convincing evidence" for automatic suspension in all cases 1
• The safer approach is checking renal function after the procedure and withholding metformin if renal function deteriorates 1
• In patients with pre-existing renal failure, metformin should preferably be stopped before the procedure 1

For contrast procedures, metformin discontinuation is standard practice 1:

• Traditional teaching recommends suspending metformin before angiography/PCI and resuming 48 hours later with adequate renal function 1
• Alternative approach: check renal function after contrast and withhold only if deterioration occurs 1

Practical Algorithm for NPO Patients

When to Hold Metformin:

1. Elective surgery - discontinue on day of surgery 1, 2
2. Procedures with IV contrast - hold before procedure 1
3. Acute illness with NPO status - discontinue immediately 1, 2
4. Bowel preparation/colonoscopy - hold during prep and procedure 4
5. Any NPO situation with dehydration risk - discontinue 2, 3

When to Resume Metformin:

• After adequate oral intake is reestablished 2
• After confirming stable renal function (check creatinine) 1
• Typically 48 hours post-procedure for contrast studies 1
• When hemodynamic stability is confirmed 1

Common Pitfalls to Avoid

Insufficient awareness among patients and clinicians 3:

• Many patients continue metformin inappropriately during high-risk situations 3
• Clinicians must provide explicit instructions to hold metformin when ordering NPO status 3

Failure to consider cumulative risk factors 4:

• Patients on ACE inhibitors/ARBs face additional AKI risk when dehydrated 4
• Consider holding these medications concurrently during high-risk NPO periods 4

Special populations requiring extra caution 5:

• Patients with ileostomy or high-output stomas have increased dehydration risk 5
• Post-surgical patients may have unrecognized volume losses 5

Inadequate discharge counseling 5:

• Patients discharged after surgery need clear instructions about when to resume metformin 5
• Assessment of hydration status and stoma output should guide restart timing 5

Heart Failure Considerations

For patients with heart failure, the American Heart Association provides nuanced guidance 1:

• Metformin should be discontinued in unstable or hospitalized HF patients 1
• In stable HF with normal renal function, metformin may be continued 1
• Avoid metformin in cardiogenic shock or distributive shock 1

Bottom Line

The conservative approach of temporarily discontinuing metformin during NPO status is medically sound and carries minimal harm 3. The drug can be easily stopped without causing adverse glycemic consequences in the short term 3. Given the potentially fatal nature of lactic acidosis, even if rare, the risk-benefit calculation strongly favors temporary discontinuation 3. Resume only when oral intake is adequate, renal function is stable, and acute illness has resolved 1, 2.