Epidurals work by depositing local anaesthetic drugs in the path of large nerves as they enter and leave the spinal cord. This occurs in an area just outside of the spinal cord called the epidural space. Local anaesthetic drugs work by temporarily interrupting the transfer of information (like pain) along these nerves. Local anaesthetics have this effect whenever they encounter nerves. If they are injected into one's skin a patch of numbness will result. This patch represents all of the skin whose sensation is dependant upon the activity of small nerves near the injected anaesthetic. A nerve block like one given by a dentist simply means that the local anaesthetic was deposited near a large nerve. A much larger area of numbness results.

Nerves entering and leaving the spinal cord pass through the epidural space. These nerves are responsible for transmitting information about touch, temperature and pain to the spinal cord and thereafter to the brain. In the other direction, they carry commands to "movement" nerves from the brain. Local anaesthetic deposited in the epidural space moves up, down and around and is able to coat many large nerves to the legs, pelvis and abdomen. To what extent they are 'blocked' will depend upon what dose is given. The spread of a dose of local anaesthetic injected through a catheter within the epidural space is shown in the illustration below. Note that the principal effects of the drug occur in the region of the emerging spinal nerves. This is in contrast to the site of action of epidural opiates.

The potential exists for the local anaesthetic to affect nerves high up in the chest which control breathing and heart rate. This could happen if too big a dose was given or if the epidural needle was inadvertently placed inside the spinal sac (which requires a much smaller dose). Epidurals have the potential to cause serious and life threatening effects like this and this is why specialist anaesthetic doctors perform them. They have proved to be very safe as a result of this.