After systemic drug administration the drug concentrations in all parts of the body are governed by the pharmacokinetic principles.

The drug is distributed to all parts of the body including the site of action where it can produce its therapeutic effects.

The intensity of the therapeutic effects and the toxic effects of most drugs are dependent on the drug cone at the site of action. Also, the drug cone in plasma is in equilibrium with the drug conc at the site of action.

This results in the high correlation between the steady state drug plasma concentration and the intensity of the therapeutic and adverse effects of the drug.