Metformin – Common Side Effects (General Information)

Very Common / Common Side Effects

Many people tolerate metformin well, but some experience side effects, especially when starting the medicine or after a dose increase. Commonly reported effects include:

• Gastrointestinal (stomach and digestion):

  • Nausea or feeling sick
  • Vomiting
  • Loose stools or diarrhoea
  • Stomach pain or cramps
  • Bloating, gas, or a feeling of fullness
  • Loss of appetite or reduced appetite
  • Metallic or unusual taste in the mouth

• General:

  • Mild tiredness or weakness in some people (but usually not severe)

Less Common Side Effects

• Skin and sensitivity:

  • Mild skin rash or itching
  • Flushing (feeling warm or red in the face)

• Vitamin B12 levels:

  • Long-term use can sometimes lower vitamin B12 levels, which may cause:

    • Numbness or tingling in hands or feet
    • Tiredness
    • Pale skin

If someone is on metformin for a long time, doctors sometimes check vitamin B12 levels as part of routine follow-up.

Rare but Serious Side Effects (Need Urgent Medical Attention)

These are uncommon but important to know. If a person on metformin develops any of the following, they should seek urgent medical attention and contact a doctor immediately:

• Signs of lactic acidosis (a rare but serious complication):

  • Feeling very weak or extremely tired
  • Unusual muscle pain
  • Breathing faster or more deeply than normal, or feeling short of breath
  • Feeling cold, dizzy, or light-headed
  • Slow or irregular heartbeat
  • Severe abdominal (stomach) pain with or without diarrhoea
  • Sudden feeling of being very unwell

• Signs of an allergic reaction:

  • Swelling of face, lips, tongue, or throat
  • Difficulty breathing or swallowing
  • Severe rash, hives, or itching

If any of these serious symptoms appear, it is important to get emergency medical help.

When to Talk to a Doctor

• If common side effects (like nausea or diarrhoea) are strong, last more than a few days, or interfere with eating or daily life.
• If there is unexplained weight loss, extreme tiredness, or symptoms of low vitamin B12 (numbness, tingling, weakness).
• Before taking any other new medicines, supplements, or herbal products alongside metformin.

Only a doctor who knows the full medical history can decide whether metformin is suitable, whether the dose is appropriate, or whether it should be changed or stopped.

For educational and informational purposes only. Not a diagnosis or medical advice. Please consult a licensed healthcare professional.

This is general drug information — do not self-medicate. Consult your doctor for personalized medical advice.

If you need more help, I’m here to assist. For medical concerns, always consult a licensed healthcare professional.