QUOTE : half-life. Preparations with high lipid solubility, such as diazepam and alprazolam, are absorbed rapidly from the GI tract and distribute rapidly to the brain by passive diffusion along a concentration gradient, resulting in a rapid onset of action. However, as the concentration of the medication increases in the brain and decreases in the bloodstream, the concentration gradient reverses itself, and these medications leave the brain rapidly, resulting in fast cessation of drug effect.