Therapeutic drugs are used to heal, control, or prevent certain diseases and conditions, and are often required throughout a person’s lifetime to treat chronic conditions. Most therapeutic drugs have a narrow therapeutic index and require regular monitoring to ensure correct levels of the drug in the blood are maintained; therapeutic index refers to the difference in the amount that causes a therapeutic effect, and the amount that causes toxicity. Toxic levels may occur as a result of other factors affecting a person’s ability to absorb the drug i.e. change in health, additional medication that may be reacting with the drug, and, overdose. In addition, sub-therapeutic levels may occur where the patient’s level of the drug is inadequate for their treatment.