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This article waslast modified on 30 May 2023.
At a Glance
Why Get Tested?

To find out if the phenobarbital level in the blood is at an appropriate level for an individual patient

When To Get Tested?

At regular intervals to monitor phenobarbital levels in the blood and if needed to detect low or toxic levels

Sample Required?

A blood sample taken from a vein in your arm

Test Preparation Needed?

No test preparation is needed

On average it takes 7 working days for the blood test results to come back from the hospital, depending on the exact tests requested. Some specialist test results may take longer, if samples have to be sent to a reference (specialist) laboratory. The X-ray & scan results may take longer. If you are registered to use the online services of your local practice, you may be able to access your results online. Your GP practice will be able to provide specific details.

If the doctor wants to see you about the result(s), you will be offered an appointment. If you are concerned about your test results, you will need to arrange an appointment with your doctor so that all relevant information including age, ethnicity, health history, signs and symptoms, laboratory and other procedures (radiology, endoscopy, etc.), can be considered.

Lab Tests Online-UK is an educational website designed to provide patients and carers with information on laboratory tests used in medical care. We are not a laboratory and are unable to comment on an individual's health and treatment.

Reference ranges are dependent on many factors, including patient age, sex, sample population, and test method, and numeric test results can have different meanings in different laboratories.

For these reasons, you will not find reference ranges for the majority of tests described on this web site. The lab report containing your test results should include the relevant reference range for your test(s). Please consult your doctor or the laboratory that performed the test(s) to obtain the reference range if you do not have the lab report.

For more information on reference ranges, please read Reference Ranges and What They Mean.

What is being tested?

This test measures the level of phenobarbital in the blood. Phenobarbital is a barbiturate, a sedating drug that depresses the nervous system. Doctors usually prescribe phenobarbital to prevent fits or to relieve anxiety. Phenobarbital may be prescribed to treat epilepsy and other seizure disorders because the drug stabilizes electrical activity in the brain. Patients may become dependent on phenobarbital after prolonged use.

Your doctor will monitor your response to phenobarbital to make sure that you have the desired level of the drug in your bloodstream and to find out what is the best dose for you. Monitoring the level also helps assure your doctor that the level of the drug circulating in your bloodstream is not toxic. Your doctor might request a phenobarbital level when you begin taking the drug, and any time while on the drug, to find out if the dose is right for you. Your doctor might also decide to request a test if you begin taking another medicine because several common drugs can affect how your body responds to phenobarbital. The following drugs can interact with phenobarbital:

  • oral blood-thinning medications like warfarin
  • antidepressants
  • central nervous system depressants, sedatives, hypnotics and tranquillisers
  • antihistamines
  • alcohol
  • oral contraceptives
  • corticosteroids like prednisone
  • doxycycline, used to treat bacterial infections
  • griseofulvin, used to treat fungal infections
  • phenytoin, another medication often prescribed to treat seizures.

It is important for you to maintain a constant level of phenobarbital in the blood that is within the “target” range (see below). If your level is too low, you may experience fits or anxiety. If the level is too high, you could experience side effects or toxicity .

Maintaining a constant, therapeutic level of phenobarbital in the blood can be difficult. Phenobarbital is metabolised by liver enzymes and removed in the urine at different rates, depending on your age and overall health. Depending on dose, age and health, full removal of the drug can take several days to weeks. Once the body has reached its capacity to metabolise 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.

Accordion Title
Common Questions
  • How is it used?

    A doctor requests the test to measure the amount of phenobarbital in the blood and to find out whether the drug level is within a target range. A doctor may request the test every few weeks when you first begin taking the drug to help adjust the dose so that you have the desired level in your blood. After this phase,, the doctor may request the test at regular intervals. If you begin taking another medicine, your doctor will usually request more measurements because phenobarbital interacts with several other common drugs. Phenobarbital might increase or decrease a specific drug’s effectiveness, and other medications may also affect the level of phenobarbital in the blood. The result could be decreased effectiveness (from too low a level) or severe side effects/toxicity (if the level is too high).

  • When is it requested?

    A doctor will usually request the test after you start phenobarbital treatment and if you begin taking or discontinue another medication that affects phenobarbital levels. Once you have a stable blood level of phenobarbital in the therapeutic range, then your doctor may monitor levels at regular intervals to ensure the levels are stable.

    A doctor might also request phenobarbital levels if you do not appear to be responding to ensure that you are taking the drug as directed or when you experience side effects or toxicity.

    Minor side effects from phenobarbital may include:

    • drowsiness
    • headache
    • dizziness
    • depression
    • excitement (especially in children)
    • upset stomach
    • vomiting.

    Tell your doctor if any of these symptoms are severe or do not go away:

    • nightmares
    • increased dreaming
    • constipation
    • joint or muscle pain.

    If you experience any of the following symptoms, telephone your doctor immediately:

    • seizures
    • mouth sores
    • sore throat
    • easy bruising
    • bloody nose
    • unusual bleeding
    • fever
    • difficulty breathing or swallowing
    • skin rash.


  • What does the test result mean?

    The target range for adults taking phenobarbital is 15-40 mg/L (60-180 micromol/L). Within this range, most people will respond to the drug without having symptoms of toxicity. However, each patient’s response to the drug and side effects is individual. You may experience side effects even with blood levels at the low end of the therapeutic range or continue to have fits at the upper end. As with other antiepileptic drugs, your doctor will work with you to find the dosage that works best for you.  Tolerance to the effects of the drug develops after you have been taking it for some time, which makes monitoring the drug concentration in the blood less useful than with some other drugs, as a given blood concentration may produce less of an effect with time. 

    You should take phenobarbital exactly as your doctor 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 fit and can affect the levels of your other medications. Always consult your doctor if you are having problems taking phenobarbital.

  • Is there anything else I should know?

    Phenobarbital has been used to treat epilepsy since the early 20th century and is still widely prescribed worldwide, despite development of many newer and less toxic agents. Because the drug may cause sedation in adults and behavioural disturbances and excitement, it is now often a second- or third-line medication in developed countries.

    It is advisable to stay on a product from a specific manufacturer, as phenobarbital preparations from different manufacturers may not be equivalent.

    In pregnancy, phenobarbital is associated with an increased risk of congenital malformations, and if you are taking phenobarbital and planning a pregnancy, or think you may be pregnant, you should discuss your epilepsy medication urgently with your doctor.

    Phenobarbitone is contra-indicated in patients with acute porphyria and should be avoided in children and in patients with a history of alcohol or drug abuse.

  • 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 doctor to know how well the medicine works to control your seizures. How long this takes will be different for each person. 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 at intervals as long as you are taking the medication. Phenobarbital, like other anti-epileptic drugs, is usually taken every day for your lifetime. An exception to this may be patients whose seizures are caused by a temporary condition; they may only need the medication for a short period of time.

  • Will this medicine affect other medicines that I am taking?

    Be sure to tell your doctor 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.