Every drug has a sub-clinical concentration (a concentration at which effective therapy would not be achieved) and a toxic concentration (a concentration at which the drug will harm the patient.) The range between the minimum effective and toxic concentrations is significant for some drugs. These drugs are thus relatively safe. It would be hard to overdose (or under-dose) with such a drug. However, some drugs have a very narrow therapeutic window and need closer monitoring. It is for these kinds of drugs that TDM was developed.
For example, the anticonvulsant carbamazepine (Tegretol) has a narrow therapeutic window. Too low of a dose will put the patient at risk of seizure. Too high of a dose can cause severe conditions such as agranulocytosis.
