1. Within the spinal canal or within a sheath.
2. Within a sheath, for example, cerebrospinal fluid that is contained within the dura mater. It also refers to drugs administered into the cerebrospinal fluid bathing the spinal cord and brain.
3. Within either the subarachnoid or the subdural space.

An intrathecal injection (often simply called "intrathecal") refers to an injection into the spinal canal (intrathecal space surrounding the spinal cord), as in a spinal anaesthesia or in chemotherapy. This route is also used for some infections, particularly post-neurosurgical infections.