The blood-brain barrier is the fatty tissue that protects the brain from viruses and toxins. Psycho-active drugs can cross the blood brain barrier because they are lipid soluble. Lipid soluble is when the drug dissolves in fat instead of water. A lipid is a fat. This is considered to be fatty so it absorbs permits fat molecules to enter. The moment a drug is absorbed it goes directly to the blood brain barrier. Scientifically, the blood-brain barrier is a result of special cells in the central nervous system that wrap themselves around the capillaries and block the pores through which stances normally diffuse.