There is no drug depot.
Psychoactive drugs are the ones that do make it into brain.
The half-life of a drug is the time it takes for half the dose to be eliminated from the body. That many drugs are sold in just a few dosages belies the fact that there is wide individual variability in drug response. All drugs produce multiple effects. Side-effects are effects not sought by the user. One person s desired effect may be another person s side effect .