Glucose is a polar molecule that relies on GLUT-1 glucose transporters to mediate facilitated diffusion across the BBB. GLUT-1 deficiency secondary to a mutation in the SLC2A1 gene can lead to frequent seizures in the first months of life. 

Gases and water readily diffuse into the brain. Very small, non-polar, lipid soluble molecules (ethanol, nicotine, diazepam) can also enter the brain by simple diffusion.