Metformin Side Effects
Common Gastrointestinal Side Effects
The most frequent side effects of metformin are gastrointestinal symptoms including bloating, abdominal discomfort, and diarrhea, occurring in approximately 53% of patients for diarrhea and 26% for nausea/vomiting. 1, 2
Diarrhea is the most common adverse effect, reported in 53% of patients in clinical trials, compared to 12% with placebo 2
Nausea and vomiting occur in 26% of patients versus 8% with placebo 2
Flatulence affects 12% of patients compared to 6% with placebo 2
Abdominal discomfort and indigestion occur in 6-7% of patients 2
These gastrointestinal symptoms can be mitigated by gradual dose titration and using the extended-release formulation rather than immediate-release 1, 3
Most GI symptoms are mild, transient, and self-limiting, typically resolving with continued use 4
Taking metformin with meals further reduces gastrointestinal side effects 4
Serious Adverse Effects
Lactic Acidosis
Lactic acidosis is the most serious complication of metformin, though extremely rare, occurring in approximately 0.03 cases per 1000 patient-years with a mortality risk of 0.015 per 1000 patient-years. 5
The occurrence of lactic acidosis is very rare in appropriate patients 1, 3
Metformin-associated lactic acidosis (MALA) carries mortality rates approaching 50% when it does occur 6
Risk is primarily elevated when eGFR is <30 mL/min/1.73 m² 1
For patients with eGFR 30-45 mL/min/1.73 m², there is increased risk due to potential fluctuations below 30 mL/min/1.73 m² 1
Warning signs of lactic acidosis include: 2
- Unusual muscle pain (myalgias)
- Malaise and unusual weakness
- Unusual sleepiness or somnolence
- Unexplained hyperventilation or trouble breathing
- Unexplained stomach/intestinal problems with nausea, vomiting, or diarrhea
- Feeling cold, especially in arms and legs
- Dizziness or lightheadedness
- Slow or irregular heartbeat
High-risk conditions for lactic acidosis: 2, 5
- Kidney impairment (serum creatinine >130 micromol/L or >1.5 g/L)
- Liver disease
- Congestive heart failure requiring treatment
- Excessive alcohol consumption
- Dehydration from illness, fever, vomiting, or diarrhea
- Age ≥80 years without kidney function testing
- Acute conditions: heart attack, severe infection, stroke
- Procedures requiring injectable contrast agents
Vitamin B12 Deficiency
Metformin use is associated with increased risk of vitamin B12 deficiency and potential worsening of neuropathy symptoms, requiring periodic monitoring of B12 levels. 1
In clinical trials, approximately 7% of patients developed subnormal vitamin B12 levels after 29 weeks of treatment 2
The mechanism involves interference with B12 absorption in the intestines 1
Periodic testing of vitamin B12 levels is recommended during long-term metformin therapy 1, 2
This is particularly important given the potential for worsening neuropathy symptoms 1
Other Notable Adverse Effects
Hypoglycemia Risk
Metformin does not cause hypoglycemia when used as monotherapy 1, 4, 5
Hypoglycemia can occur when metformin is combined with sulfonylureas or insulin 2
In toxicity cases, hypoglycemia occurred in 2.8% of exposures, more common than previously recognized 7
Additional Reported Effects
Less common adverse effects (1-5% incidence) include: 2
- Asthenia (weakness) - 9%
- Headache - 6%
- Abnormal stools
- Myalgia
- Lightheadedness
- Dyspnea
- Rash and increased sweating
- Taste disorder
- Chest discomfort
Hepatic Effects
Cholestatic, hepatocellular, and mixed hepatocellular liver injury have been reported in postmarketing surveillance 2
These are rare but should be monitored in patients with pre-existing liver disease 5
Reproductive Considerations
- Metformin may induce ovulation in premenopausal anovulatory women, potentially leading to unintended pregnancy 2