Anesthesia enables the painless performance of medical procedures that would cause severe or intolerable pain to an unanesthetized patient. Three broad categories of anaesthesia exist:

General anesthesia suppresses central nervous system activity and results in unconsciousness and total lack of sensation.
Sedation (or dissociative anesthesia) inhibits transmission of nerve impulses between the cerebral cortex and limbic system, which inhibits both anxiety and creation of long-term memories.
Conduction anesthesia, commonly known as regional or local anesthesia, blocks transmission of nerve impulses between a targeted part of the body and the spinal cord, which causes loss of sensation in the targeted body part. A patient under conduction anesthesia remains fully conscious. Two categories of regional anesthesia exist. A peripheral blockade inhibits sensory perception in a body part, such as numbing a tooth for dental work or administering a nerve block to stop sensation from an entire limb. A central blockade administers the anesthetic around the spinal cord, which suppresses all sensation below the block. Examples of central blockade include epidural and spinal anaesthesia.
In preparing for a medical procedure, an anesthesiologist chooses and determines the doses of one or more drugs to achieve the types and degree of anesthesia characteristics appropriate for the type of procedure and the particular patient. The types of drugs uses include general anesthetics, hypnotics, sedatives, neuromuscular-blocking drugs, narcotic, and analgesics.