Phenobarbitone blood level
Quick answer: This test measures the amount of phenobarbitone, a medicine used for seizures, in your blood. It is used to check that the dose is in an effective and safe range, and when side effects are suspected.
- SampleBlood
- TakesAbout 5 minutes
- Before the testThe sample is usually taken just before your next dose; follow your doctor's timing and do not change your dose on your own.
How the test is done
- 1You come for the test before your next dose, if your doctor asked for a trough level
- 2Staff note when you took your last dose
- 3Blood is drawn from a vein into a tube
- 4A plaster is applied and you continue your medicine as prescribed
What your results can mean
Reading your report
Your doctor compares the level with the target range for your treatment.
If the result is outside the usual range
A level above the target range can be seen with dose changes, liver problems or interacting medicines, and may cause drowsiness or unsteadiness. A level below range can be seen with missed doses or interactions and may raise the chance of seizures.
Who usually has this test
People, including babies and children, taking phenobarbitone for seizures, especially after a dose change, new medicine or breakthrough seizures.
Risks and aftercare
A small bruise is possible, and you should keep taking your medicine as prescribed unless your doctor advises otherwise.
Names and options
Also called: phenobarbital level, serum phenobarbitone
Listed by the hospital as
- PHENOBARBITONE
Questions about this test
Why does phenobarbitone need blood level checks?
The right dose differs between people, and too little or too much can cause problems. Blood levels help doctors adjust the dose, especially at the start or when other medicines change.
What are signs that the phenobarbitone level may be too high?
Too much can cause drowsiness, slurred speech, unsteady walking or confusion. If you notice these, contact your doctor.
Can phenobarbitone affect pregnancy planning?
Phenobarbitone can affect the baby and can make hormonal contraception less effective. If you are planning a pregnancy, talk to your doctor early, and do not stop or change your dose on your own.
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This page explains each test in general terms so you know what to expect. It is not medical advice: your doctor decides which tests you need and what your results mean. Reference ranges differ between laboratories. Updated 4 November 2026.