Fainting is not a disease itself, but rather a symptom of an underlying cause or condition. It may be due to harmless causes or to more serious conditions. However, in about a third of the cases, episodes of syncope may prove impossible to explain. Potential causes of fainting include:

• Hypoglycemia (low blood glucose), insulin shock (severe hypoglycemia) and other metabolic disorders
  
  
• Cardiovascular disorders, including low blood pressure, stroke, pulmonary arterial hypertension and many heart conditions

• Dehydration
  
  
• Autonomic neuropathy
  
  
• Severe forms of hyperglycemia (high glucose) that can result in diabetic coma, such as diabetic ketoacidosis or hyperosmolar hyperglycemic nonketotic syndrome (HHNS)
   
• Sudden changes in body position, especially going from sitting or lying down to standing
  
  
• Certain medications (e.g., antihypertensives, opioids, anti-inflammatories) or medical procedures (e.g., injection, blood test, tilt-table test)
  
  
• Alcohol or drug abuse
  
  
• Emotional stress
  
  
• Pain
  
  
• Hyperventilation
  
  
• Overheating
  
  
• Heavy sweating
  
  
• Exhaustion
  
  
• Allergic reactions, such as an insulin allergy
  
  
• Insufficient diet
  
  
• Anemia (insufficient amount of red blood cells)
  
  
• Neurological disorders (e.g., seizures)
  
  
• Head injury
  
  
• Psychiatric disorders, including eating disorders
  
  
• Lung disorders
  
  
• Addison’s disease (insufficient production of adrenal hormones)
  
  
• Coughing or laughing
  
  
• Urination or defecation
  
  
• Vomiting

Abnormal glucose levels, autonomic neuropathy, cardiovascular disorders and dehydration are major causes of fainting in people with diabetes. These conditions can affect the brain and nervous system and therefore may result in fainting.

Scientists recently discovered that severe, chronic fainting can be due to an autoantibody that impedes the autonomic nervous system. This condition has been resolved with a blood transfusion and medication that keeps more antibodies from forming.