When measuring blood levels guides dosing
Therapeutic drug monitoring means measuring a drug's concentration in the blood to help adjust the dose, rather than relying on the dose amount alone. It is most useful for drugs where the gap between an effective level and a toxic level is narrow, where the response is hard to judge clinically, and where blood concentration correlates well with effect. Examples often cited include certain antiepileptics, some antibiotics, lithium, and digoxin.
For most everyday medicines monitoring adds little, because the safety margin is wide and the effect can be observed directly.
Why timing of the sample matters
Levels are usually interpreted against a target range, and the sample is most often a trough drawn just before the next dose, because trough levels are reproducible and reflect the lowest concentration during the interval. Some drugs also need a peak level, and samples drawn before steady state is reached can mislead.
A reported number means little without knowing when it was taken relative to the dose. This is general educational information, not medical advice; interpreting drug levels and changing doses must be done by a qualified clinician.