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 doctor 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.
A doctor 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 doctor 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 doctor 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 doctor if any of the following persist or become severe:
The therapeutic range for adults taking phenobarbital is 20-40 micrograms/milliliter (mcg/ml) (86-173 micromole/liter). Within this range, most people will respond to the drug without displaying symptoms of toxicity. However, each patient'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 antiepileptic drugs, the doctor will work with the person who is being treated to find the dosage that works best.
Phenobarbital has been used to treat epilepsy since the early 20th century and is still the most widely prescribed antiepileptic 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 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 seizure and can affect the levels of your other medications. Always consult your doctor if you are having problems taking phenobarbital.
Your doctor 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
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
This article was last reviewed on November 27, 2012. | This article was last modified on November 27, 2012.
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