Therapeutic drug monitoring is a branch of clinical chemistry that specialises in the measurement of medication levels in blood. Its main focus is on drugs with a narrow therapeutic index, i.e. drugs that can easily be under- or overdosed.

In pharmacology, many medications are used without monitoring of blood levels, as their dosage can generally be varied according to the clinical response that a patient gets to that substance. In a small group of drugs, this is impossible, as insufficient levels will lead to undertreatment or resistance, and excessive levels can lead to toxicity and tissue damage.

Examples of drugs analysed by therapeutic drug monitoring:

• Aminoglycoside antibiotics (gentamycin)
• Antiepileptics (such as phenytoin and valproic acid)
• Mood stabilisers, especially lithium citrate
• Antipsychotics (such as pimozide and clozapine)

Therapeutic drug monitoring can also detect poisoning with above drugs, should the suspicion arise.