The drug and metabolite plasma concentration-time profiles after a single and/or multiple drug administration are dependent on the pharmacokinetic parameters of the drug and the metabolites.

When the metabolites possess pharmacological activity or can produce serious adverse effects, it is important to monitor both the drug and the metabolite conc.

The plasma concentrations of procainamide (parent drug) and N-acetylprocainamide (metabolite) are monitored in patients who receive procainamide. This is because N-acetylprocainamide possesses antiarrhythmic effect.

The plasma concentrations of carbamazepine (parent drug) and carbamazepine epoxide (metabolite) are monitored in patients who receive carbamazepine. This is because carbamazepine epoxide is responsible, at least partially, for carbamazepine toxicity.