鞘内(intrathecal、<intra- + theca、IT、"within a sheath">、脊髓内、鞘内注射、脊髓腔内注射)指某些东西"发生在"或"引入到"鞘内部(一般而言指脑或脊髓蛛网膜(arachnoid mater)、在其下是蛛网膜下腔<subarachnoid space>)的解剖空间(Spatium)或潜在空间(potential space)之情状。 例如："鞘内免疫球蛋白"(intrathecal immunoglobulin)，以及"鞘内注射"(intrathecal injection、通常简称为"鞘内"<an intrathecal>)等。"鞘内免疫球蛋白"的制造是产生在脊髓的抗体里。 而"鞘内注射"是让"药物"通过给药途径注射到椎管，更具体地说是进入"蛛网膜下腔"以到达脑脊液(CSF)，且用于脊髓麻醉(Spinal anaesthesia)、化学疗法，或疼痛管理(Pain management)等应用方面。这条"鞘内注射"的"给药途径"也可用于引入"某些药物"来对抗某些感染，特别是"后神经外科药物"(post-neurosurgical)。这种"后神经外科药物"需要以此"鞘内注射的给药途径"来处理以避免血脑屏障。不同于鞘内注射、口服(Oral administration (per os))亦给予"相同的药物"且一定会传入血流里，不过可能无法对大脑进行传递。鞘内注射药物的处方必须由专门的药师或技术人员处理，药物不能包含任何的防腐剂或其他可能有害的"非活性"(inactive)成分，不过有时在标准进程注射药物制剂时会发现这些情况。

Intrathecal administration is a route of administration for drugs via an injection into the spinal canal, or into the subarachnoid space so that it reaches the cerebrospinal fluid (CSF) and is useful in spinal anaesthesia, chemotherapy, or pain management applications. This route is also used to introduce drugs that fight certain infections, particularly post-neurosurgical. The drug needs to be given this way to avoid the blood brain barrier. The same drug given orally must enter the blood stream and may not be able to pass out and into the brain. Drugs given by the intrathecal route often have to be made up specially by a pharmacist or technician because they cannot contain any preservative or other potentially harmful inactive ingredients that are sometimes found in standard injectable drug preparations.