TDM is not useful for these drugs or in these specific situations:

• Intracelluar drugs that need to be converted to active forms (like AZT)
• 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 where continued use can lead to tolerance such that the levels needed for pain relief in one person would be toxic to another person