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What is being tested?

This test measures the level of phenobarbitone (or phenobarb) in the blood. Phenobarb is a barbiturate, a highly addictive, sedating drug that depresses the nervous system. Doctors usually prescribe phenobarb to stop or prevent seizures. Phenobarb is often prescribed to treat epilepsy and other seizure disorders because the drug stabilises electrical activity in the brain. A related drug, primidone, is also used for people with seizure disorders and phenobarb levels may be tested in people taking this medication.


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

  • other medications used to treat seizures such as phenytoin and valproate (sodium valproate)
  • oral anticoagulant medications like warfarin
  • antidepressants
  • central nervous system depressants, sedatives, hypnotics and tranquilisers
  • antihistamines
  • alcohol
  • oral contraceptives
  • corticosteroids like prednisone
  • doxycycline used to treat bacterial infections
  • griseofulvin, a drug used to treat fungal infections


It’s important for you to maintain a constant level of phenobarb in the blood within the therapeutic range. If your level is too low, you may experience seizures. If the level is too high, you could experience side effects or even toxicity.


But maintaining a constant, therapeutic level of phenobarb in the blood can be difficult. Phenobarb is metabolised by liver enzymes and eliminated in the urine at different rates, depending on your age and overall health. Depending on dose, age and health, elimination can take several days to weeks.

How is it used?

A doctor orders the test to measure and monitor the amount of phenobarb 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 phenobarb interacts with several other common drugs. Phenobarb might increase or decrease a specific drug’s effectiveness, and that medication may also affect the level of phenobarb 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 order the test after you start phenobarb treatment and if you begin taking or discontinue another medication that affects phenobarb levels. Once you have a stable blood level of phenobarb in the therapeutic range, then your doctor will monitor levels at regular intervals to ensure the levels are stable.


A doctor might also order phenobarb 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 phenobarb 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 result mean?

The therapeutic range for adults taking phenobarbitone is about 65-170 umol/L (15-40 mg/L) [your local laboratory may quote a slightly different range]. 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 phenobarb 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. Stopping the drug suddenly may cause withdrawal symptoms. Always consult your doctor if you are having problems taking phenobarbitone.

Is there anything else I should know?

Phenobarb has been used to treat epilepsy since the early 20th century but because the drug causes sedation and other side effects, it is now often a second or third line medication in developed countries like Australia.

Common questions

  • 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 optimisation include how often you have seizures, what other medicines you may be taking, and how your body responds to phenobarb.


  • How long will my phenobarb level have to be monitored?

In general, your levels will need to be monitored as long as you are taking the medication. Phenobarb, 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.


  • 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 phenobarb works, lowering or raising the level in your blood. Phenobarb may also affect how other medicines work.

More information

RCPA Manual: Phenobarbitone

Last Updated: Thursday, 1st June 2023

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