TDM is also not useful in these specific situations:
- For drugs which act intracellularly, or that need to be converted to active forms (like AZT).
- For drugs in which the effects last much longer than the serum concentrations of the drugs; examples include antineoplastics (cancer chemotherapies) and warfarin.
- Narcotic pain medications in which continued use can lead to tolerance such that the levels needed for pain relief in one person would be toxic to another person.