The blood–brain barrier (BBB) inhibits most medications from reaching the brain. The epithelial-like tight junctions inside the brain capillary endothelium are responsible for this feature. The BBB is physically and physiologically separate from the choroid plexus' blood–cerebrospinal fluid barrier. The blood-brain barrier (BBB) is a dynamic structure that acts as a gatekeeper and reflects the brain's specific requirements. The presence of the BBB makes developing novel medicines for brain diseases or new radiopharmaceuticals for brain imaging challenging. Biotechnology produces only large molecule medications that do not cross the BBB. While it is commonly considered that small molecules travel freely through the BBB, only about 98 percent of all small molecules do so.