The vagus nerve is one of 12 cranial nerves. It is the longest of the cranial nerves, extending from the brainstem to the abdomen by way of multiple organs including the heart, esophagus, and lungs. Also known as cranial nerve X, the vagus forms part of the involuntary nervous system and commands unconscious body procedures, such as keeping the heart rate constant and controlling food digestion.

Electrical stimulation of the vagus nerve, called vagus nerve stimulation (VNS), is sometimes used to treat people with epilepsy or depression.

The vagus nerve is involved in one of the most common causes of fainting, called vasovagal syncope. This is an overreaction of the body to certain stimuli, like the sight of blood, which involves the stimulation of the vagus nerve. This stimulation causes a drop in blood pressure and heart rate. Less blood flows to brain, resulting in loss of consciousness. In most cases, vasovagal syncope does not require treatment.