Where is an intrathecal injection administered?

An intrathecal injection is administered into the subarachnoid space, which is located within the meninges of the central nervous system. This space lies between the arachnoid mater and the pia mater, surrounding the spinal cord and brain. The purpose of this injection is to deliver medications directly into the cerebrospinal fluid (CSF), allowing for rapid and efficient distribution throughout the central nervous system.

Delivering medication intrathecally is particularly useful for treatments where systemic delivery would be less effective or where the medication needs to act directly on the central nervous system. For example, administering anesthetics or certain chemotherapy agents via this route can provide localized effects with potentially fewer systemic side effects.

In contrast, administering an injection into brain tissue, the spinal cord, or the cerebral cortex would not achieve the same distribution of medication as the subarachnoid space. Therefore, the subarachnoid space is the specifically targeted area for intrathecal injections, making it the correct answer.