Choosing an appropriate dosing regimen for drugs with wide (large ) is easier because therapeutic drug conc can be achieved with wide range of dosing regimens.
MTC
MEC
All the three dosing regimens
maintain therapeutic drug conc all
the time at steady state.
Drugs with narrow require accurate selection of the dosing regimen to maintain therapeutic plasma conc.
MTC
MEC
Toxic concentration
Subthepeutic concentration
Small changes in the dosing regimen may lead
to subtherapeutic or toxic concentrations.
b- Drugs with large inter-patient variability in their pharmacokinetic parameters.
The correlation between the plasma drug conc and the administered dose is very poor for this group of drugs.
It is more difficult to determine the drug dose required to achieve therapeutic drug conc when there is large inter-subject variability in the pharmacokinetic parameters.
c- Drugs administered to patients who are at high risk due to concurrent diseases or due to multiple drug therapy.
These patients require accurate selection of their dosing regimen to produce rapid therapeutic effect and to avoid serious toxicity.
IMPORTANT
Drugs that have pharmacologically active metabolites (e.g. procainamide), or toxic metabolites (e.g. carbamazepine) require monitoring the parent drug as well as the metabolite plasma conc.
Drugs that are highly bound to plasma protein and/or their protein binding is variable (e.g. phenytoin) may require monitoring the free (unbound) drug conc.
