Fainting: What Causes It ? Fainting can happen when not enough oxygen flows through your blood and into your brain. You lose consciousness, or "pass out," for a very brief time--just a few minutes.
First Aid - Fainting Spell The loss of consciousness is usually brief, and the episode can have no medical significance or it can be a symptom of a serious disorder. Therefore, treat loss of consciousness as a medical emergency until the symptoms are relieved and the cause is known.