But if it happens more than once, you should tell your doctor about it. If you faint when you turn your head to the side, the bones in your neck may be pinching on one of the blood vessels that leads to your brain. If this happens to you, be sure to tell your doctor about it. A drop in your blood sugar may also cause you to faint. This can happen if you have diabetes, but it may also happen if you don't eat for a long time.
Some prescription medicines can cause fainting.
Be sure to talk to your doctor if you think your fainting may be related to a medicine you're taking. Alcohol, cocaine and marijuana can also cause fainting. More serious causes of fainting include seizures and problems with the heart or with the blood vessels leading to the brain. Your doctor will probably want to talk to you about exactly what was happening when you fainted. He or she may ask you for details about how you felt right before and right after you fainted.