This article was last reviewed on
This article waslast modified on May 13, 2019.
At a Glance
Why Get Tested?

To monitor the phenobarbital level in your 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 someone 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?


You may be able to find your test results on your laboratory's website or patient portal. However, you are currently at Lab Tests Online. You may have been directed here by your lab's website in order to provide you with background information about the test(s) you had performed. You will need to return to your lab's website or portal, or contact your healthcare practitioner in order to obtain your test results.

Lab Tests Online is an award-winning patient education website offering information on laboratory tests. The content on the site, which has been reviewed by laboratory scientists and other medical professionals, provides general explanations of what results might mean for each test listed on the site, such as what a high or low value might suggest to your healthcare practitioner about your health or medical condition.

The reference ranges for your tests can be found on your laboratory report. They are typically found to the right of your results.

If you do not have your lab report, consult your healthcare provider or the laboratory that performed the test(s) to obtain the reference range.

Laboratory test results are not meaningful by themselves. Their meaning comes from comparison to reference ranges. Reference ranges are the values expected for a healthy person. They are sometimes called "normal" values. By comparing your test results with reference values, you and your healthcare provider can see if any of your test results fall outside the range of expected values. Values that are outside expected ranges can provide clues to help identify possible conditions or diseases.

While accuracy of laboratory testing has significantly evolved over the past few decades, some lab-to-lab variability can occur due to differences in testing equipment, chemical reagents, and techniques. This is a reason why so few reference ranges are provided on this site. It is important to know that you must use the range supplied by the laboratory that performed your test to evaluate whether your results are "within normal limits."

For more information, please read the article Reference Ranges and What They Mean.

What is being tested?

Phenobarbital is a long-acting barbiturate, an anti-epileptic drug (AED) and sedating drug that depresses the nervous system. Healthcare practitioners 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. This test measures the level of phenobarbital in the blood.

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. The toxic effects include drowsiness, confusion, and lack of voluntary coordination of muscle movement (ataxia), which can affect things like driving performance. Those who are chronically treated may also develop tolerance to the sedative effects of the drug and become dependent.

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.

Furthermore, phenobarbital is broken down (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 healthcare provider 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 health practitioner 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 practitioner 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 anti-epileptic medications such as phenytoin. In addition, phenobarbital may affect the metabolism of the above co-administered drugs and dose adjustment may needed.

Accordion Title
Common Questions
  • How is it used?

    This test is used to measure and monitor the amount of phenobarbital in the blood and to determine whether the drug level is within a therapeutic range. Phenobarbital is an anti-epileptic drug used to prevent seizures in people with epilepsy. This test is used to ensure that the blood level of phenobarbital is not too low so as to cause a recurrence of seizures or too high as to cause side effects. Depending on the result, a healthcare practitioner may adjust the drug dose up or down.

    If the person being treated begins taking another medication, this test may be used to monitor the blood level because phenobarbital interacts with several other common drugs. Phenobarbital might increase or decrease a specific drug's effectiveness, and that medication may also affect the level of phenobarbital in the blood. The result could be decreased effectiveness from a level that is too low or severe side effects/toxicity if the level is too high.

  • When is it ordered?

    A healthcare practitioner will usually order the test after the start of phenobarbital treatment and every few weeks thereafter. Testing may be more frequent any time the dose is adjusted up or down. Once a stable blood level of phenobarbital is achieved and is in the therapeutic range, then the health practitioner will typically monitor levels at regular intervals to ensure that they are stable.

    More frequent testing may be done if an individual begins taking or discontinues another medication that affects phenobarbital levels.

    A healthcare provider might also order phenobarbital levels if the person does not appear to be responding to the medication and continues to experience seizures.

    Testing may be ordered when an individual experiences side effects from phenobarbital. Most side effects are not serious and often go away with no further action. A person should notify their healthcare provider if any of the following persist or become severe:

    • Drowsiness
    • Headache, dizziness
    • Depression
    • Excitement (especially in children)
    • Upset stomach, vomiting
    • Nightmares, increased dreaming
    • Constipation
    • Joint or muscle pain

    If any of the following symptoms are present, the healthcare provider should be notified immediately:

    • Seizures
    • Mouth sores, sore throat
    • Easy bruising, bloody nose, unusual bleeding
    • Unexplained fever
    • Difficulty breathing or swallowing
    • Severe skin rash
  • What does the test result mean?

    The therapeutic range for adults taking phenobarbital is 15-40 micrograms/milliliter (mcg/ml) (65-173 micromole/liter) for seizure treatment, and 5-15 micrograms/milliliter (mcg/ml) (22-65 micromole/liter) for sedative-hypnotic use. The therapeutic range for seizure treatment in children (younger than 5 years old) is narrower: 15-30 micrograms/milliliter (mcg/ml) (65-130 micromole/liter).

    Within these ranges, most people will respond to the drug without displaying symptoms of toxicity. However, each person's response to the drug and side effects is individual. A person may experience side effects even with blood levels at the low end of the therapeutic range or continue to have seizures at the upper end. As with other anti-epileptic drugs, the healthcare provider will work with the person who is being treated to find the dosage that works best.

  • Is there anything else I should know?

    Phenobarbital has been used to treat epilepsy since the early 20th century and is still the most widely prescribed anti-epileptic drug worldwide, despite development of several others since then. Because the drug causes sedation and other side effects, it is now often a second- or third-line medication in developed countries. But phenobarbital is still a first-line drug in many developing nations.

    You should take phenobarbital exactly as your healthcare provder has prescribed it. Do not decrease the dose, increase it, or discontinue the medication on your own because doing so can increase your risk of having a seizure and can affect the levels of your other medications. Always consult your healthcare provider if you are having problems taking phenobarbital.

    Your healthcare provider might decide to order a test if you begin taking another medication because several common drugs can affect how your body responds to phenobarbital. The following drugs can have effects with phenobarbital:

    • Oral blood-thinning medications like warfarin
    • Antidepressants and tricyclics, including MAO inhibitor antidepressants
    • Central nervous system depressants, sedatives, hypnotics and tranquilizers
    • Antihistamines
    • Alcohol
    • Oral contraceptives
    • Corticosteroids like prednisone
    • Doxycycline used to treat bacterial infections
    • Griseofulvin, a drug used to treat fungal infections
    • Phenytoin, another medication often prescribed to treat seizures
  • How long will it take for the medicine to work?

    It may take a number of weeks to find the right dose, and even more time for your healthcare provider to know how well the medicine works to control your seizures. How long this takes will be different for each person. It may require 14-21 days to achieve the steady therapeutic blood concentrations. Factors affecting dose optimization include how often you have seizures, what other medicines you may be taking, and how your body responds to phenobarbital.

  • How long will my phenobarbital level have to be monitored?

    In general, your levels will need to be monitored as long as you are taking the medication. Phenobarbital, like other anti-epileptic drugs, is usually taken every day (sometimes several times a day) for your lifetime. Exceptions to this may be adults who have outgrown their epileptic syndromes from childhood or people whose seizures are caused by a temporary condition and only need the medication for a limited time.

  • Will phenobarbital affect other medicines that I am taking?

    It might. Be sure to tell your healthcare providers the names of all prescription medicines, herbal or dietary supplements, vitamins, and over-the-counter medicines you take. Some of these may interfere with how phenobarbital works, lowering or raising the level in your blood. Phenobarbital may also affect how other medicines work.

  • I am taking the drug primidone. Why is my doctor monitoring phenobarbital?

    Primidone is another medication that is used to control seizures. Phenobarbital is the major breakdown product (metabolite) of primidone that is also active in controlling seizures. To ensure that there is enough active drug in your blood to control your symptoms without causing toxicity, your healthcare provider may monitor phenobarbital when you are prescribed primidone.

View Sources

Sources Used in Current Review

June 2015 review performed by Irene Shu, PhD, DABCC, Assistant Laboratory Director, USDTL.

(2015 July 1, Updated) Mayo Clinic. Barbiturate. Available online at through Accessed on 7/13/2015.

Suzanne Bentley, MD (2014 January 3rd, Updated). Phenobarbital Level. Medscape. Available online at through Accessed on 7/13/2015.

Baselt R.C. (© 2014). Disposition of Toxic Drugs and Chemicals in Man. 10th Edition: Biomedical Publications. Seal Beach, CA. Pp 1607-1609.

Levine B. I. (© 2010). Principles of Forensic Toxicology 3rd Edition: American Association for Clinical Chemistry, Inc. Washington, DC. Pp 215 – 220.

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.

(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.

Ask a Laboratory Scientist

This form enables you to ask specific questions about lab tests. Your questions will be answered by a laboratory scientist as part of a voluntary service provided by one of our partners, American Society for Clinical Laboratory Science. Please allow 2-3 business days for an email response from one of the volunteers. 

Thank you for using the Consumer Information Response Service ("the Service") to inquire about the meaning of your lab test results.  The Service is provided free of charge by the American Society for Clinical Laboratory Science, which is one of many laboratory organizations that supports Lab Tests Online.
Please note that information provided through this free Service is not intended to be medical advice and should not be relied on as such. Although the laboratory provides the largest single source of objective, scientific data on patient status, it is only one part of a complex biological picture of health or disease. As professional clinical laboratory scientists, our goal is to assist you in understanding the purpose of laboratory tests and the general meaning of your laboratory results. It is important that you communicate with your physician so that together you can integrate the pertinent information, such as age, ethnicity, health history, signs and symptoms, laboratory and other procedures (radiology, endoscopy, etc.), to determine your health status. The information provided through this Service is not intended to substitute for such consultations with your physician nor specific medical advice to your health condition.
By submitting your question to this Service, you agree to waive, release, and hold harmless the American Society for Clinical Laboratory Science and its affiliates or their past or present officers, directors, employees, agents, and Service volunteers (collectively referred to as "ASCLS") and the American Association  for Clinical Chemistry and its affiliates or their past or present officers, directors, employees, agents, and Service volunteers (collectively referred to as "AACC") from any legal claims, rights, or causes of action you may have in connection with the responses provided to the questions that you submit to the Service.
AACC, ASCLS and its Service volunteers disclaim any liability arising out of your use of this Service or for any adverse outcome from your use of the information provided by this Service for any reason, including but not limited to any misunderstanding or misinterpretation of the information provided through this Service.