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The blood-brain barrier (BBB) is a selective, semipermeable membrane between the blood and the interstitium of the brain, allowing cerebral blood vessels to regulate the movement of molecules and ions between the blood and the brain.[1] The BBB is composed of endothelial cells, pericytes, capillary basement membrane, and astrocyte end-feet, all of which aim to shield the brain from toxic substances, filter harmful compounds from the brain to the bloodstream, and supply brain tissue with nutrients. To do this, the BBB has physical (tight junctions) and metabolic (enzyme) barriers.[2] Central nervous system (CNS) structures are unique in structure and function and therefore require a stable environment with a composition that differs from that of the peripheral circulation. For this reason, the BBB exists to maintain a homeostatic environment in which CNS structures can function without disruption from other bodily functions.