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
The Test Sample
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 is often prescribed to treat epilepsy and other seizure disorders because the drug stabilizes electrical activity in the brain. Patients may become dependent on the drug 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 have effects 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, a drug used to treat fungal infections
- phenytoin, another medication often prescribed to treat seizures.
It is important for you to have a constant level of phenobarbital in the blood that is within the “target” range. 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 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.
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
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. Afterwards a period of time, the doctor will probably 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 that medication 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, call your doctor immediately:
- seizures
- mouth sores
- sore throat
- easy bruising
- bloody nose
- unusual bleeding
- fever
- difficulty breathing or swallowing
- severe skin rash.
What does the test result mean?
The target range for adults taking phenobarbital is 15-40 mg/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.
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 fits 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 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 remains a first-line drug in many developing nations.
Common Questions
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Article Sources
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.
