What is the Most Serious Side Effect of Metformin

What is the Most Serious Side Effect of Metformin

Metformin, a common medication prescribed to manage type 2 diabetes, has transformed countless lives by aiding in blood sugar regulation. While this drug has proven beneficial for many, it’s not without side effects. Let’s explore the most serious side effect of metformin and what you should be aware of.

Metformin: A Brief Overview

Before diving into the side effects, it’s essential to understand metformin’s purpose. Belonging to a drug class known as biguanides, metformin functions by:

  • Decreasing glucose production in the liver.
  • Increasing sensitivity to insulin.
  • Reducing the amount of sugar absorbed from food.

By performing these actions, metformin keeps blood sugar levels stable, helping to mitigate the risks associated with type 2 diabetes.

Common Side Effects of Metformin

Some side effects of metformin are relatively mild and are often expected when beginning the medication:

  • Stomach upset
  • Diarrhea
  • Nausea and vomiting
  • Metallic taste
  • Skin rash

These symptoms generally decrease over time as the body adjusts to the medication. However, it’s the more severe side effects that require immediate attention.

Lactic Acidosis: The Most Serious Side Effect

Lactic acidosis is the most severe side effect associated with metformin. It’s a rare but critical condition where the body produces excess lactic acid, lowering the blood’s pH. This can result in a host of severe symptoms and even be life-threatening if not treated promptly.

Symptoms of Lactic Acidosis include:

  • Unusual fatigue or weakness.
  • Breathing difficulties.
  • Unexplained muscle pain.
  • Feeling cold, especially in the extremities.
  • Dizziness or lightheadedness.
  • Slowed heart rate.
  • Stomach pain accompanied by nausea and vomiting.

Why does Lactic Acidosis Occur with Metformin?

Under normal circumstances, our cells produce small amounts of lactic acid. Typically, the liver and kidneys manage and excrete this acid, maintaining a balanced pH in the bloodstream. However, metformin can occasionally impede the liver’s ability to clear lactic acid, leading to its accumulation.

Several factors increase the risk of metformin-induced lactic acidosis:

  • Kidney or liver dysfunction.
  • Recent surgery or serious infections.
  • Prolonged fasting or malnutrition.
  • Excessive alcohol consumption.
  • Certain other medications taken concurrently.

Managing the Risk

To prevent lactic acidosis, it’s crucial to:

  • Undergo Regular Monitoring: Regular blood tests can track liver and kidney function, ensuring they’re effectively clearing lactic acid.
  • Adjust Dosage: Your doctor might adjust the metformin dosage based on your health status, age, or if you’re taking other medications.
  • Stay Informed: Understand the symptoms of lactic acidosis, so you can seek medical attention if they arise.


  1. How common is lactic acidosis with metformin?
    Lactic acidosis is rare. The incidence is estimated at about 3 cases per 100,000 patient-years of exposure to metformin.
  2. Should I stop taking metformin if I experience severe side effects?
    Always consult your doctor before making any changes to your medication regimen.
  3. Is there an alternative to metformin if I’m at higher risk for lactic acidosis?
    Yes, several alternatives are available. Discuss potential options with your healthcare provider.
  4. Does every patient on metformin experience side effects?
    No, many patients tolerate metformin well without significant side effects.


While metformin is generally safe and effective, it’s paramount to remain aware of its potential side effects, particularly lactic acidosis. Regular check-ups and clear communication with healthcare professionals can ensure a balanced and informed approach to managing type 2 diabetes.