Fainting is caused by a sudden reduction in blood flow to the entire brain. The brain stops working when blood flow stops for more than a few seconds.

Common causes of fainting include:

• Acute hyperventilation
• Dehydration
• Drug side effect
• Emotional stress
• Emphysema
• Fear
• Hypoglycemia
• Low blood pressure
• Nausea
• Orthostatic hypotension
• Pain
• Panic attacks
• Severe coughing
• Straining during a bowel movement
• Vasovagal syncope

Less common causes of fainting include:

• Abdominal aortic aneurysm
• Acute thoracic dissection
• Allergic reaction
• Aortic stenosis
• Atrial fibrillation
• Cardiac arrhythmia
• Cardiomyopathy
• Carotid artery disease
• Carotid sinus hypersensitivity
• Congenital heart disease
• Congestive heart failure
• Defecation syncope:
• Fainting during a bowel movement
• Dehydration
• Digoxin toxicity
• Electrical injury
• Gastrointestinal bleeding
• Heart attack
• Heat illness
• Hypertrophic cardiomyopathy
• Insulin reaction
• Long QT syndrome
• Lown Ganong Levine syndrome
• Micturition syncope:
• Fainting while urinating
• Phobias
• Polycythemia vera
• Pulmonary embolism
• Ruptured ectopic pregnancy
• Ruptured spleen
• Sick sinus syndrome
• Seizure
• Severe anemia
• Smoke inhalation
• Snakebite
• Stroke
• Subarrachnoid hemorrhage
• Subclavian steal syndrome
• TIA
• Third degree heart block
• Wolff Parkinson White syndrome