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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.

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. Also, some medicines used to mental health conditions. These include carbamazepine, lamotrigine, sodium valproate, clozapine, lithium and tricyclic antidepressants.

When therapeutic drug monitoring is done

Therapeutic drug monitoring may be done:

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 and your weight. It also depends on the way your body handles the medicine and whether you are taking other medicines.

Things that can cause levels to change

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

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.

Suitable times include:

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

Written by HealthInfo clinical advisers. Page created December 2023.


Page reference: 1293304

Review key: HIMMH-215644