Therapeutic drug monitoring

Therapeutic drug monitoring measures the level of a medicine in your blood. It is done to make sure the level is high enough to treat your illness effectively. It also checks that the level is not too high, which can cause side effects. The range between these levels is called the therapeutic range.

On this page

Which medicines are monitored

Monitoring is particularly important for medicines that have a small therapeutic range.

Therapeutic drug monitoring is only done for a limited number of medicines. These include some anti-seizure (anticonvulsant) medicines. It is also done for some medicines used to treat mental health conditions. These include:

  • carbamazepine
  • lamotrigine
  • sodium valproate
  • clozapine
  • lithium
  • tricyclic antidepressants.

When therapeutic drug monitoring is done

Therapeutic drug monitoring may be done:

  • when you first start a medicine to find out when the therapeutic level has been reached
  • if you change the dose or time that you take the medicine.

The blood sample is taken several days after you start the medicine or change its dose. This is because it takes time for the medicine to build up in your blood and reach a level that is effective for you. This level is called the steady state.

The time taken to reach the steady state depends on several factors. These include:

  • the type of medicine
  • your age
  • your weight
  • the way your body handles the medicine
  • whether you are taking other medicines.

Therapeutic drug monitoring may also be done:

  • at regular intervals to make sure a therapeutic level is being maintained
  • if you get unusual side effects, which indicate the possibility of toxic levels
  • if the medicine is not working as expected.

The best time to have your blood sample taken

It is important to take your blood sample at the right time.

The blood sample is usually taken just before you are due to take your next dose. This is when the level of the medicine in your body is at its lowest. This is called the trough level.

  • If you are taking your medicine once a day in the evening, have the blood test more than 12 hours after your last dose. For example, late morning or in the afternoon.
  • If you are taking your medicine once a day in the morning, withhold the morning dose and have the blood test first thing in the morning. Then take your medicine after the test.
  • If you are taking your medicine twice a day in the morning and evening, withhold the morning dose and have the blood test first thing in the morning. Then take your medicine after the test.

For some medicines, your healthcare provider will give you more specific instructions about when to have the blood test.

Things that can cause levels to change

The level of a medicine in your body can change due to:

  • adding a new medicine to the medicines you are already taking
  • getting older or becoming ill
  • forgetting to take medicines or not taking the correct dose of the medicine
  • for some medicines (for example, clozapine), stopping or starting smoking.