Fig. 3

Schematic illustration of the different routes of drug administration’ pathways to the brain. The intranasal route is the only route that provides a direct pathway to the brain avoing the BBB (green arrow), along with an indirect pathway (red arrow). The remaining routes reach the brain indirectly (red arrows) via the systemic blood circulation passing via the BBB. Oral, in particular, and rectal route undergo first-pass hepatic metabolism, although rectally administered drugs could potentially avoid the first-pass metabolism, if they do not reach more cranial parts of the colon. Figures from authors’ personal record modified with microsoft power point