Metformin Side Effects
The principal side effects of metformin are gastrointestinal intolerance (bloating, abdominal discomfort, diarrhea), vitamin B12 deficiency with potential neuropathy, and very rarely, lactic acidosis—particularly in patients with renal impairment. 1
Common Gastrointestinal Effects
The most frequent adverse reactions are gastrointestinal and occur in the majority of patients:
- Diarrhea affects approximately 53% of patients 2
- Nausea/vomiting occurs in about 26% 2
- Flatulence (12%), indigestion (7%), and abdominal discomfort (6%) are also common 2
These symptoms can be substantially mitigated by gradual dose titration and using the extended-release formulation rather than immediate-release. 1 Women tend to experience gastrointestinal symptoms more frequently than men 3. These effects are typically mild and transient, though they lead to discontinuation in approximately 6% of patients 2.
Vitamin B12 Deficiency
Metformin causes vitamin B12 deficiency through interference with B12 absorption from the B12-intrinsic factor complex in the gastrointestinal tract. 2
Clinical Impact:
- Approximately 7% of patients develop subnormal vitamin B12 levels in clinical trials of 29-week duration 1, 2
- Both daily dose and duration of use are independent risk factors for deficiency 4
- The deficiency is associated with elevated homocysteine and methylmalonic acid levels 4
- Most critically, vitamin B12 deficiency can worsen symptoms of neuropathy 1 and in severe cases cause subacute combined degeneration of the spinal cord 5
Monitoring Requirements:
Measure hematologic parameters annually and vitamin B12 levels every 2-3 years in all patients on metformin. 2 The 2025 ADA guidelines explicitly recommend periodic testing of vitamin B12 levels 1. High-risk populations requiring more vigilant monitoring include the elderly, vegetarians/vegans, and patients on multiple medications 4.
The deficiency appears rapidly reversible with either discontinuation of metformin or vitamin B12 supplementation 2. Patients with inadequate vitamin B12 or calcium intake/absorption are particularly predisposed 2.
Lactic Acidosis
Metformin-associated lactic acidosis (MALA) is very rare but potentially fatal. 1 The occurrence is primarily in patients with significant renal impairment 2.
Pathophysiology:
Metformin decreases hepatic uptake of lactate and inhibits mitochondrial oxidative phosphorylation, increasing lactate blood levels 2, 6. MALA is characterized by:
- Blood lactate >5 mmol/L 2
- Anion gap acidosis (without ketonuria/ketonemia) 2
- Metformin plasma levels generally >5 mcg/mL 2
Risk Factors and Contraindications:
Renal impairment is the primary risk factor. 2 Specific recommendations:
- Contraindicated when eGFR <30 mL/min/1.73 m² 2
- Do not initiate when eGFR is 30-45 mL/min/1.73 m² 2
- Assess benefit vs. risk if eGFR falls below 45 mL/min/1.73 m² during therapy 2
- Check eGFR at least annually; more frequently in elderly patients 2
Additional contraindications and high-risk situations: 2
- Active or progressive severe liver disease
- Active alcohol abuse (alcohol potentiates metformin's effect on lactate metabolism)
- Acute/unstable heart failure, hypoperfusion, hemodynamic instability
- Conditions causing hypoxemia (cardiovascular collapse, acute MI, sepsis)
- Age ≥65 years (increased risk of hepatic, renal, or cardiac impairment)
Contrast Imaging Precautions:
Stop metformin at the time of or prior to iodinated contrast procedures in patients with: 2
- eGFR 30-60 mL/min/1.73 m²
- History of hepatic impairment, alcoholism, or heart failure
- Intra-arterial contrast administration planned
Re-evaluate eGFR 48 hours after imaging and restart only if renal function is stable 2.
Clinical Presentation and Management:
MALA presents with nonspecific symptoms: malaise, myalgias, abdominal pain, respiratory distress, increased somnolence 2. Severe cases show hypotension and resistant bradyarrhythmias 2.
Treatment requires immediate metformin discontinuation and prompt hemodialysis (metformin is dialyzable with clearance up to 170 mL/min) 2. Hemodialysis often results in symptom reversal and recovery 2.
Other Adverse Effects
Less common reactions (1-5% incidence) include hypoglycemia (when combined with insulin or sulfonylureas), myalgia, lightheadedness, dyspnea, rash, increased sweating, and taste disorder 2. Rare postmarketing reports include hepatocellular, cholestatic, and mixed liver injury 2.
Metformin itself does not cause hypoglycemia, but increases the risk when combined with insulin or insulin secretagogues—requiring dose reduction of these agents. 2