To determine if the phenobarbital level in the blood is at a therapeutic level for an individual patient
Phenobarbital
Phenobarbital
Também conhecido como
[Often referred to by brand name (see MedlinePlus Drug Information)]
Nome formal
Phenobarbital
Este artigo foi revisto pela última vez em
Este artigo foi modificado pela última vez em 10 de Julho de 2017.
At a Glance
Why Get Tested?
When To Get Tested?
At regular intervals to monitor phenobarbital level in the blood and as needed to detect low or toxic levels
Sample Required?
A blood sample drawn from a vein in your arm
Test Preparation Needed?
None
Accordion Title
Common Questions
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How is it used?A doctor orders the test to measure and monitor the amount of phenobarbital in the blood and to determine whether the drug level is within a therapeutic range. A doctor may order 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, the doctor will probably order the test at regular intervals. If you begin taking another medication, your doctor will likely order more monitoring 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.
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When is it ordered?A doctor will usually order 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 will monitor levels at regular intervals to ensure the levels are stable.
A doctor might also order 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.
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What does the test result mean?The therapeutic range for adults taking phenobarbital is 20-40 mcg/ml (86-173 micromol/L). 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. 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 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 seizure and can affect the levels of your other medications. Always consult your doctor if you are having problems taking phenobarbital.
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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 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.
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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.
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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. 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.
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Will this medicine affect other medicines that I am taking?Be sure to tell your health care 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.