Medication monitoring measures the level of a medicine in the blood to make sure the dosage of medicine is correct. Sometimes the amount of medicine that helps (therapeutic level) is very close to the amount that can cause harm (toxic level). Determining the correct dosage of a medicine for each person can be hard, because the dose needed to produce a therapeutic level in the blood varies from person to person. Things that affect medicine levels in the blood include age, weight, activity level, the speed at which the body breaks down the medicine, how the medicine was taken (such as by mouth, patch, or shot), and other medicines a person is using.

Medication monitoring is important for people whose bodies do not remove or break down (metabolize) medicines well because of poor function in the liver or kidneys. Also, some medicines can damage an organ (such as the liver or kidneys) or cause other problems at the dose used for treatment. If you are taking such a medicine, its amount in your blood may need to be checked regularly to ensure your dosage is at a safe and effective level.