Drug metabolism involves modification of the chemical structure of the administered drug to form a new chemical entity known as the metabolite. The metabolites have their own pharmacokinetic parameters which are different from the pharmacokinetic parameters of the parent drug.

The maximum amount of the metabolite formed can be equal to the dose of the parent drug in terms of molar amounts. However the plasma metabolite concentrations can be higher or lower than the parent drug concentrations depending on the metabolite pharmacokinetic parameters.