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Also known as: [Often referred to by brand name (see MedlinePlus Drug Information)]
Formal name: Phenobarbital

At a Glance

Why Get Tested?

To monitor the phenobarbital level in the blood to ensure a therapeutic level while avoiding toxic side effects

When to Get Tested?

At the start of therapy and at a regular intervals during treatment; when an individual has signs and symptoms of toxicity or is experiencing seizures

Sample Required?

A blood sample drawn from a vein in your arm

Test Preparation Needed?


The Test Sample

What is being tested?

This test measures the level of phenobarbital in the blood. Phenobarbital is a barbiturate, an antiepileptic drug (AED) and sedating drug that depresses the nervous system. Doctors usually prescribe it to prevent seizures or to relieve anxiety. It is often prescribed to treat epilepsy and other seizure disorders because the drug stabilizes electrical activity in the brain.

It is important to maintain a stable level of phenobarbital in the blood within the therapeutic range. If the level is too low, the person who is being treated may experience seizures or anxiety. If the level is too high, the individual could experience side effects or even toxicity.

Maintaining a constant, therapeutic level of phenobarbital in the blood can be difficult. The difference between the level at which the drug is therapeutic and the level at which toxic side effects can occur is very small. This is called a narrow therapeutic index and is a primary reason the drug requires close monitoring.

Futhermore, phenobarbital is metabolized by liver enzymes and eliminated in the urine at different rates, depending on a person's age and overall health. Depending on dose, age and health, elimination can take several days to weeks. Once the body has reached its capacity to metabolize phenobarbital, small increases in dose can result in large increases in levels of the drug in the blood. Side effects can become more severe, and toxicity may occur.

A doctor will monitor an individual's response to phenobarbital to make sure that the desired level of the drug is maintained in the blood and to determine the dose that works best for the person treated. The doctor might order a phenobarbital level at the start of treatment and any time while the person is on the medication to determine if the dose is right. The doctor might also decide to order a test if a person begins taking another medication because several common drugs can affect how the body responds to phenobarbital. Drugs that can have effects with phenobarbital include warfarin, antidepressants, sedatives, hypnotics, tranquilizers, antihistamines, alcohol, oral contraceptives, corticosteroids like prednisone, and other antiepileptic medications such as phenytoin.

How is the sample collected for testing?

A blood sample is collected by inserting a needle into a vein in the arm.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

The Test

Common Questions

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Article Sources

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.

Sources Used in Current Review

(September 27 2012) National Institutes of Health's Medline Plus. Phenobarbital. Available online at Accessed October 2012.

(September 25, 2012) Ochoa J. Antiepileptic Drugs. Medscape Reference. Available online at through Accessed October 2012.

(December 2, 2011) Cavasos J. Epilepsy and Seizures. Medscape Reference. Available online at through Accessed October 2012.

(April 28, 2011) Mayo Clinic. Epilepsy Treatment and Drugs. Available online at through Accessed October 2012.

Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. Burtis CA, Ashwood ER, Bruns DE, eds. 4th edition, St. Louis: Elsevier Saunders; 2006, Pp 1252-1253.

Sources Used in Previous Reviews

National Institutes of Health's MedlinePlus. Phenobarbital. Available online at Reviewed October 1, 2007. Accessed October 3, 2008. Phenobarbital. Available online at through Reviewed August 1, 2007. Accessed October 3, 2008.

Warner, A. et al. Standards of laboratory practice: Antiepileptic Drug Monitoring. Clinical Chemistry 1998; 44:1085–1095. Available online at through Accessed October 4, 2008.

Kwan, P. and Brodie, M. Phenobarbital for the Treatment of Epilepsy in the 21st Century: A Critical Review. Epilepsia 2004; 9: 1141-1149. Available online at through Accessed October 4, 2008.

American Society of Health System Pharmacists. Phenobarbital (Systemic). Available online at through Revised August 2007. Accessed October 5, 2008.

San Diego Reference Laboratory. Phenobarbital. Available online at through Accessed October 7, 2008.

Burgess, C. ed. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 4th Edition, 2006. P. 2312.

Epilepsy Therapy Development Project. Children and Phenobarbital. Available online at through Accessed October 5, 2008.

Emory School of Medicine, Department of Human Genetics. Seizures and Pregnancy. PDF available for download at through Issued 2004. Accessed October 7, 2008.

UC San Diego Medical Center. Safety of Commonly Used Drugs in Nursing Mothers. Available online at through Reviewed April 2005. Accessed October 8, 2008.

Spencer, Jeanne P. et al. American Family Physician. Medications in the Breast Feeding Mother. Published July 1, 2001. Available online at through Accessed October 8, 2008.

Food and Drug Administration. Information for Healthcare Professionals: Suicidality and Antiepileptic Drugs. Available online at through Issued January 31, 2008. Accessed October 4, 2008.

Reinberg, Steve. FDA Advisers Don't Back 'Black Box' Warning for Epilepsy Drugs. The Washington Post. Available online at through Published July 10, 2008. Accessed October 7, 2008.