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Phenobarbital

Medicine levels · also printed as Phenobarbital level, Phenobarbitone, Phenobarbitone level, Serum phenobarbital

Typical UK adult range15 to 40 mg/L

The lab prints the reference band; many people are well controlled outside it and the treating team's judgement applies.

What this test measures

Phenobarbital is an anti-epilepsy medicine (some are also used for mood or pain). The blood level helps check that enough is being absorbed and that the amount is not building up.

Why doctors request it

Checked when seizures are not controlled, when side effects occur, in pregnancy, or when another medicine has been added.

What commonly pushes it up

Kidney or liver disease
Some medicines as a class that slow its breakdown
A sample taken too soon after the tablet
Low albumin, for phenytoin and valproate, which changes the free fraction

What commonly pushes it down

Missed tablets
Some medicines as a class that speed its breakdown
Pregnancy, for lamotrigine especially

Everyday reasons for a value outside the range

Trough timing matters
Many people are well controlled with levels outside the printed band, so the level is read with the seizure history

About the sample

Just before the next tablet where possible.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.

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This is an educational summary of results you uploaded. It is not medical advice, a diagnosis or a treatment plan, and it does not replace the doctor who requested the test. If you are worried, or a result is marked outside range, discuss it with your doctor. Written and reviewed by a UK consultant physician. Reference ranges vary between laboratories.