Every drug has a sub-clinical concentration (a concentration at which effective therapy won't be achieved) and a toxic concentration (a concentration at which the drug will be harmful to the patient.)
For some drugs, the range between the minimum effective concentration and the toxic concentration is large. These drugs are thus relatively safe. It would be hard to overdose (or under-dose) with such a drug. But 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 very narrow therapeutic window. Too low of a dose will have the patient at risk of seizure. Too high of a dose can cause serious conditions such as agranulocytosis.
