Medication Monitoring

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Medication Monitoring

Therapeutic Index of Medications
Monitoring the Effects of Anticoagulants
Monitoring Anticonvulsant Drugs
Risks of Medication Monitoring


Medication monitoring is the process of monitoring prescription medications to check that blood levels are within the expected and desired range, or the process of measuring the effects of a prescription medication in the blood.


Medication monitoring is done by measuring blood levels of a drug or some aspect of a drug’s effects. Some prescription drugs are very beneficial for health and may even be life saving, yet are very dangerous if even a small overdose is given. Because of this danger, drugs that are associated with severe or life threatening medical effects after a small overdose would not be available for use in medicine without a method by which to monitor their blood levels or their effects. Medication monitoring is a critical tool in the administration of these medications.

Therapeutic Index of Medications

All drugs may be dangerous if taken in great enough quantity. However for some drugs, the difference between a therapeutic dose and a dangerous or poisonous dose is smaller than others. The figurative “distance” between a medically effective and safe dose of a medication and its toxic dose is known as the therapeutic index or therapeutic window. Drugs with wide therapeutic windows require a greatly increased dose before they are toxic and are therefore safer to use than drugs with narrow therapeutic windows. Drugs with narrow therapeutic windows have a smaller difference in dose amount between their beneficial and harmful effects. If the therapeutic index of a drug is small enough, that drug may require medication monitoring techniques to be added as part of the treatment regimen to ensure that it is used safely.

Monitoring the Effects of Anticoagulants

Anticoagulant drugs are administered to inhibit the ability of the blood to form a blood clot, a process known as blood coagulation. Anticoagulants are not administered to dissolve existing blood clots, but rather to prevent the clotting process from forming a new blood clot or increasing an existing one. The most common health condition that may require anticoagulation drug therapy is a cardiac arrhythmia (abnormal heart rhythm) called atrial fibrillation. Atrial fibrillation is a rapid arrhythmia in a portion of the heart known as the atria, that may lead to a stroke. A stroke is a condition involving the inability of blood vessels in the brain to provide oxygen to brain tissue. A stroke may be caused by a blood clot blocking the blood vessel that supplies a part of the brain, thereby depriving the brain of oxygen and causing death of brain tissue. In atrial fibrillation, blood clots are sometimes knocked off of the heart tissue and travel through the blood to the brain, causing a stroke. Anticoagulant therapy may prevent a stroke by preventing the formation of blood clots. It may also prevent blood clots that could damage heart tissue or lead to a heart attack. The anticoagulant drug warfarin (coumadin) is most commonly used for this purpose.

While anticoagulant drugs have great benefit for prevention of stroke in atrial fibrillation, they can also be very dangerous. If the blood is kept too “thin” (via a decreased ability to clot), small wounds or a bump on the head may cause life threatening internal bleeding. Under normal conditions, these minor injuries would not cause excessive bleeding because the blood would clot and close the wound. With too great a dose of warfarin, minor injuries may become catastrophes. Even without injury, dangerous internal bleeding such as gastrointestinal bleeding or brain hemorrhages may occur. For this reason, the amount of warfarin given must only be the minimum necessary for treatment.

There is no one dose of warfarin that is safe and standardized for all patients. It must be individually determined, and the amount that is necessary may change frequently even within an individual. The effects of warfarin must be monitored at least weekly


  • Which of my medications needs to be routinely monitored?
  • Why does this medication require monitoring?
  • Why is this medication the best treatment option for my health condition?
  • What are the effects of too much of this medication?
  • How often do I need to come have my blood levels checked?
  • Is the drug being measured in my blood, or an effect of the drug?

to ensure that too much has not been given (avoiding dangerous blood conditions) or that too little has been given to provide the therapeutic effect. Warfarin is not directly measured from blood samples, but rather the blood is examined for the degree of change in its blood clotting mechanisms. By measuring the effect of warfarin on the blood, doses can be tailored up or down to maintain a therapeutic but safe level. There is a very narrow window in which warfarin’s effects are desirable, and it is very close to dangerous doses. Without medication monitoring techniques, warfarin could not be used to save lives in patients with heart conditions.

Monitoring Anticonvulsant Drugs

Anticonvulsant drugs are used to treat epilepsy, a type of neurological seizure disorder. The goal of anti-convulsant therapy is to provide seizure prevention without causing too many adverse side effects of drug treatment. Anticonvulsants such as phenytoin (dilantin) are beneficial for seizure patients, but also have a relatively narrow therapeutic index. Additionally, there is great variability in the way different patients metabolize the drug. Studies have shown that different patients taking the same dose of phenytoin may have up to a 50-fold difference in blood levels. There is also variability in the amount of drug needed to control seizure disorders in different patients within a safe range of doses. For all these reasons, there is no one dose that is standardized for all patients. Rather, the dose used for each patient varies widely, and needs to be individually tailored and monitored. The level of phenytoin may be directly measured in blood serum to make sure that blood levels are within the therapeutic range, and that they are not toxic levels. If blood levels of phenytoin are below what is considered a therapeutic range, doses


Atria— Plural for atrium.

Atrial Fibrillation— A dangerous heart condition involving a rapid, irregular heartbeat.

Atrium— Chamber of the heart that contains specialized heart cells that set the heartbeat.

Blood Serum— The fluid portion of the blood.

Cardiac Arrhythmia— An irregular heartbeat.

Coagulation— Blood clotting.

Epilepsy— A neurological disorder that involves uncontrolled seizures.

Gastrointestinal Tract— The path in the body from the mouth, through the stomach, intestines, rectum, and the anus.

Phlebitis— Inflammation of a vein.

Stroke— An event causing impairment of blood circulation to the brain, causing death of brain tissue and potentially drastically affecting mental functioning.

are usually increased within safe limits even if the patient has not been experiencing seizures since initiating therapy.

Anticonvulsant drugs may be monitored when a patient first starts the drug to make sure they reach therapeutic levels, after a patient has been on the drug to make sure they are maintaining therapeutic and not toxic levels, or to explore symptoms of toxicity that may potentially be caused by the drug. Anticonvulsant drugs may also be monitored when the patient is experiencing seizures despite drug therapy. If levels of the current anticonvulsant are within the established therapeutic range of doses, then there may be need to assess whether to add other medications to the treatment regimen or switch to another medication entirely.

Risks of Medication Monitoring

There is very little risk associated with having blood drawn for medication monitoring. Most people have no side effects or a small bruise. However, with any blood draw there is a small chance that the area around the punctured vein may develop phlebitis, the inflammation of a vein. Phlebitis may also involve a bacterial infection if the site of the blood draw was not appropriately cleaned before the needle was inserted. Phlebitis can be locally painful but usually resolves in a short period of time.



Kumar, Vinay, Nelson Fausto, and Abul Abbas. Robbins & Cotran: Pathologic Basis of Disease, Seventh Edition. Saunders, Elsevier, 2005.


“Anticoagulant Drugs.” American Heart Association. October 22, 2007. [Accessed April 15, 2008].

“Stroke.” American Heart Association. [Accessed April 15, 2008].

Olson, K. R. “Toxicity, Warfarin and Superwarfarins.” eMedicine. January 17, 2008. [Accessed April 15, 2008].

Maria Basile, PhD

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Medication Monitoring

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