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Blood Test Tracker

Phenobarbital

Measures phenobarbital, a long-acting seizure medicine, in your blood. It helps your team see whether your dose gives a steady level in the usual range.

AI-verified against cited sources. Awaiting expert reviewAlso called PHBLOINC 3948-7

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What it is

Phenobarbital is a long-acting barbiturate used to treat seizures and, in some settings, as a sedative. It leaves the body slowly, with an average half-life of about 79 hours in adults, so levels change gradually and stay fairly steady through the day. This test measures how much phenobarbital is in your blood. Your prescriber compares the result with a usual range to see whether your dose suits you.

Why it matters

Phenobarbital levels vary a lot between people on the same dose, and liver function, age and other medicines can change them. A blood level helps your team check that you are getting the medicine as prescribed, look into possible side effects such as drowsiness or unsteadiness, and see whether a dose change had the expected effect. The number is only a guide. How you feel and how well your seizures are controlled matter more than the number alone.

If it’s low

A low level means less phenobarbital is in your blood than the usual range. This can happen after missed doses or a dose reduction, or when other medicines change how fast it is cleared. A low level doesn't always mean seizures will return, since some people are controlled at lower levels.

Common causes

  • Missed or late doses
  • A recent dose reduction
  • Sample drawn before the level reached its steady value after a dose change
  • Other medicines that change how fast phenobarbital is cleared

What can help

  • Tell your prescriber if you have missed doses or have trouble taking the medicine as planned
  • Ask whether your other medicines could be affecting your level

If it’s high

A high level means more phenobarbital is in your blood than the usual range. At higher levels, drowsiness, slowed thinking, unsteady walking and rapid eye movements (nystagmus) become more likely, and very high levels can slow breathing. People who have taken it for a long time can tolerate higher levels than others, so the number needs context.

Common causes

  • A recent dose increase, since levels take weeks to reach their new steady level
  • Reduced liver function, because the liver clears most of the drug
  • Medicines that slow its breakdown
  • Primidone, which the body partly converts to phenobarbital
  • Taking extra doses by mistake

What can help

  • Ask your prescriber how this result fits with how you have been feeling
  • Tell your prescriber about every medicine, supplement and any alcohol you use
  • Ask whether your liver tests should be checked

Usual ranges

What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.

For adults: 15–40 mcg/mL. Source: ARUP Laboratories, Scientific Reports, Perspectives in Clinical Research. Labs treat above 60 mcg/mL as urgent.

What can affect your result

  • Time since starting or changing the dose, because steady state takes weeks
  • Liver function, since the liver clears most of the drug
  • Other medicines that change how fast phenobarbital is cleared
  • Primidone, which is partly converted to phenobarbital
  • Age: the half-life in newborns and children differs from adults (FDA labeling)
  • The laboratory's method and printed range
  • Liver function, age and other medicines change how fast phenobarbital is cleared, so the same dose can give different levels in different people.
  • Primidone is partly converted to phenobarbital in the body, so people taking primidone have phenobarbital in their blood.
  • Different laboratories use different methods and ranges, so compare trends within the same laboratory when you can.

Phenobarbital leaves the body slowly, so the level changes little through the day and the time of the draw matters less than for most seizure medicines. Some labs still ask for a sample just before the next dose. Slow. With an average half-life of about 79 hours in adults (range 53 to 118), a new steady level takes roughly two weeks or more after starting or changing the dose.

When to talk to a clinician

Contact your prescriber promptly if you notice marked drowsiness, confusion, unsteady walking or slurred speech, or if seizures become more frequent. Talk with your prescriber before changing or stopping phenobarbital. If someone may have taken too much, can't be woken, has trouble breathing, or has a seizure that doesn't stop, call 911 or your local emergency number (in the US, Poison Control is 1-800-222-1222).

Questions you could ask

  • What level are you aiming for in my case?
  • How often should my phenobarbital level be checked?
  • Could my other medicines or my liver function be raising my level?
  • Are my side effects, such as drowsiness, related to this level?
  • Could missed doses or other medicines explain a low level?
  • Should we recheck the level after a few weeks?

Questions people ask

What is a normal phenobarbital level?

For people taking phenobarbital, many labs print 15 to 40 mcg/mL (some print 10 to 40) as the usual range. If you don't take it, the level should be very low or not measurable. Your prescriber looks at the number together with your seizure control and side effects.

Does the time of day of my blood draw matter?

Less than for most seizure medicines. Phenobarbital leaves the body slowly, so the level changes little during the day. Your lab may still ask for a sample before your next dose.

How do I convert mcg/mL to µmol/L?

Multiply mcg/mL by 4.31, so 15 mcg/mL is about 65 µmol/L and 40 mcg/mL is about 172 µmol/L. To go the other way, multiply µmol/L by 0.2322.

Show the science and sources

Units

  • 1 mcg/mL = 1 mcg/mL
  • 1 mcg/mL = 1 mg/L
  • 1 mcg/mL = 4.306 µmol/L

Open the Phenobarbital unit converter

For clinicians and researchers

Phenobarbital is a long-acting barbiturate cleared mainly by hepatic metabolism, with CYP2C9 the main enzyme and smaller roles for CYP2C19, CYP2E1 and UGTs. Protein binding is low (about 20 to 45%), so total and free concentrations track each other more closely than for phenytoin. FDA labeling for phenobarbital tablets gives a mean adult half-life of 79 hours (range 53 to 118) and 110 hours (range 60 to 180) in children and newborns under 48 hours old. It induces hepatic microsomal enzymes, which changes the levels of many other drugs. The label quotes usual lethal blood levels of 100 to 200 mcg/mL and notes that tolerance with long-term use complicates interpretation of a level. ARUP prints a reference range of 15 to 40 mcg/mL for ages 3 months and older, matching one published hospital list, and a Western India clinic table lists 10 to 40 mcg/mL as its range. A 2026 survey of 411 US hospitals found a median critical limit of 60 mcg/mL (range 40 to 100). 1 mcg/mL equals 4.306 µmol/L (molecular weight 232.23). Barbiturate screens on urine are a separate qualitative test.

LOINC 3948-7 (primary), 14874-2, 114382-5, 114381-7, 60468-6. Specimen: serum-or-plasma.

Sources

  1. Drug Critical Limits for Urgent Physician Notification, Clinical Pharmacology & Therapeutics (2026)
  2. Identification and characterization of critical values in therapeutic drug monitoring: a retrospective analysis, Scientific Reports (2024)
  3. Unveiling the recent trends: Therapeutic drug monitoring of antiepileptic drugs in Western India's Tertiary Care Hospital, Perspectives in Clinical Research (2025)
  4. Phenobarbital (test 0090230), ARUP Laboratories
  5. Phenobarbital tablets: prescribing information, Sun Pharmaceutical Industries (DailyMed) (2026)
  6. Sezaby (phenobarbital sodium) for injection: prescribing information, U.S. Food and Drug Administration (DailyMed) (2025)
  7. Effect of hypoalbuminemia on drug pharmacokinetics, Frontiers in Pharmacology (2025)
  8. Phenobarbital (PubChem CID 4763), PubChem, National Library of Medicine

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

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