Headache

• Headaches are a common and minor side effect of taking metformin, and can be treated with pain relievers as directed by your doctor.
  
  

Digestive

• Metformin may cause digestive side effects including stomachaches, nausea, vomiting, diarrhea and constipation.
  
  

Dizziness

• Feeling dizzy or lightheaded is an uncommon side effect of metformin that should be evaluated by your doctor.
  
  

Tiredness

• Unusual weakness of your muscles, fatigue and drowsiness, and a general ill feeling are side effects of metformin that require medical evaluation.
  
  

Cardiac

• Cardiac side effects of taking metformin include irregular heartbeat and chest pain, which require urgent medical evaluation.
  
  

Allergic Reaction

• Although rare, allergic reactions to metformin may occur and include symptoms such as hives, itching, shortness of breath, and swelling of the face, mouth or tongue.
  
  

Lactic Acidosis

• Lactic acidosis is a life-threatening side effect of metformin caused by a buildup of lactic acid in your blood, and requires emergency medical care.