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 . Side effects may be mild or disturbing or even dangerous. When side-effects cause problems they are often called adverse reactions . An allergic response to a drug after it has been used more than once.