Patient Test Information


  • 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?


  • 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 phenobarbital level in the blood.

    When you are being treated with phenobarbital, it is important to maintain a stable level of phenobarbital in the blood within the therapeutic range. If the level is too low, you may experience seizures or anxiety. If the level is too high, you 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. If you are treated long-term, you may also develop tolerance to the sedative effects of the drug and become psychologically or physically dependent.

    Maintaining a constant, therapeutic level of phenobarbital in the blood can be difficult. The gap 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 your age and overall health. For different dose, age and health, elimination can take several days to weeks. Once the body has reached its maximum capacity to metabolize phenobarbital, even small increases in dose can result in large increases of the drug level in the blood. Side effects can become more severe and toxicity may occur.

    Your healthcare provider will monitor your 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 you. Your healthcare practitioner might order a phenobarbital level at the start of treatment and any time while you are on the medication to determine if the dose is right. The practitioner might also decide to order a test if you begin taking another medication because clinically significant drug interactions may occur.

  • How is the test 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. This test is used to ensure that the blood level of phenobarbital is not too low to cause a recurrence of seizures or too high to cause side effects. Depending on the result, your healthcare practitioner may adjust the drug dose up or down.

    Your healthcare practitioner might decide to order a phenobarbital level if you begin taking another medication because drug interactions can affect how your body responds to phenobarbital. The following drugs are reported to have clinically significant interactions 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
    • Estradiol, estrone, progesterone and other steroidal hormones

    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 healthcare practitioner will typically monitor levels at regular intervals to ensure that they are stable.

    More frequent testing may be done if you begin taking or discontinue another medication that affects phenobarbital levels.

    A healthcare provider might also order phenobarbital levels if you do not appear to be responding to the medication and continue to experience seizures.

    Testing may be ordered when you experience side effects from phenobarbital. Most side effects are not serious and often go away with no further action. You 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. You 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, your healthcare provider will work with you to find the dosage that works best.

    Is there anything else I should know?

    Keep all appointments with your healthcare provider and the laboratory. Your healthcare practitioner will order certain lab tests to check your response to phenobarbital.

    Do not let anyone else take your medication. Ask your pharmacist any questions you have about refilling your prescription.

    It is important for you to keep a written list of all of the prescription and nonprescription (over-the-counter) medicines you are taking, as well as any products such as vitamins, minerals, or other dietary supplements. You should bring this list with you each time you visit a healthcare provider or if you are admitted to a hospital. It is also important information to carry with you in case of emergencies.

    You should take phenobarbital exactly as your healthcare provider has prescribed it. Do not decrease, increase the dose, 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.

    How long will it take for the medicine to work?

    It may take several 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 will this take depends on the conditions of each person. It may require 14-21 days to achieve the steady therapeutic blood level. 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

    Current review performed by Yanhua Zhang, PhD, DNRCC-TC, FAACC, Scientific Director, Total Toxicology Labs.

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