Intrathecal administration is only appropriate for a limited number of drugs (Scurr [227]):

The advantage of this route is that it allows the direct access to the CNS of a drug that does not normally cross the blood–brain barrier in sufficient amounts and thus ensures constant levels of the drug in this area. The main disadvantage is that it requires a standard lumbar puncture before the drug can be injected, and this may need to be performed on a daily to weekly basis (Stanley [239], Wilkes [268]). Although this can be quick and easy to perform, it can be distressing for the patient and could even result in CNS trauma and infection. It may also only reach the epidural or subdural spaces and therefore the concentrations in the ventricles may not be therapeutic (Wilkes [268]). However, central instillation of the drug into the ventricle can be achieved via an Ommaya reservoir (Figure 23.9), which is surgically implanted through the cranium (Sewell et al. [230], Weinstein and Hagle [265], Wilkes [268]). It carries more risks but provides permanent access and can be inserted under local or general anaesthetic (Sansivero and Barton‐Burke [216], Wilkes [268]). Doses of intraventricular drugs tend to be lower than those given intrathecally.