Phenobarbital Unit Converter
Phenobarbital Unit Converter
Convert phenobarbital between µg/mL, mg/L and µmol/L — and read the result knowing that the half-life is measured in days, so a level taken before steady state says almost nothing, and that tolerance means the quoted range guides rather than dictates.
Phenobarbital converter
Mass ⇄ molarSerum phenobarbital 20 µg/mL at steady state
The conversion
µg/mL = µmol/L ÷ 4.30589
because 4.30589 = 1 mg/L ÷ 232.24 g/mol, the molecular weight of phenobarbital
- µg/mL = mg/L
- the same concentration written two ways, so no arithmetic is needed between them. North American reports usually print µg/mL (sometimes mcg/mL); UK and European reports usually print mg/L or µmol/L
- MW 232.24
- phenobarbital, C₁₂H₁₂N₂O₃ — also spelled phenobarbitone, and marketed as the sodium salt, whose formula weight is higher but which circulates as the same molecule
- the half-life, about 4 days
- roughly 80–100 hours in adults, and longer in neonates. Five half-lives is the usual rule for steady state, so a dose change does not fully show up in the blood for two to three weeks — the single most important fact for anyone reading a phenobarbital level
- primidone
- primidone is metabolised to phenobarbital, so a patient on primidone has a measurable phenobarbital concentration. If the report is unexpected, check the prescription for primidone before concluding anything
Worked example
Serum phenobarbital 20 µg/mL at steady state
20 µg/mL = 20.0 mg/L — the same concentration
20 × 4.30589 = 86.12, which the page displays as 86 µmol/L
86 µmol/L sits inside the BNF's 15–40 mg/L range, which is 65–172 µmol/L
The two published ranges disagree at the bottom and not at the top: the ILAE reference range starts at 10 mg/L (43 µmol/L) rather than 15 mg/L (65 µmol/L). Both are guides, and neither is a target
Now the part the arithmetic cannot do. If this level was taken four days after a dose increase, it is still rising: with a half-life of about four days the patient is only halfway to their new steady state, and the same dose will eventually produce a substantially higher figure
And a patient who has taken phenobarbital for years may be entirely well at 45 µg/mL (194 µmol/L) or sedated at 18 µg/mL (78 µmol/L). Tolerance moves the individual's useful concentration away from the population band in both directions
Published ranges, side by side
| Source | µg/mL (= mg/L) | µmol/L |
|---|---|---|
| BNF, therapeutic plasma concentration, adult | 15 – 40 | 65 – 172 |
| ILAE TDM position paper, reference range, adult | 10 – 40 | 43 – 172 |
| Mayo Clinic Laboratories, adult therapeutic | 10.0 – 40.0 | 43 – 172 |
| Mayo Clinic Laboratories, infants and children | 15 – 30 | 65 – 129 |
| Mayo critical value | 60 and above | 258 and above |
Why the half-life changes how a level is read
| Time since the dose change | Proportion of the way to steady state | What a level tells you |
|---|---|---|
| 1 day | about 16% | Almost nothing about the new dose |
| 4 days (one half-life) | 50% | Roughly half the eventual rise; an apparently safe level may double |
| 8 days | 75% | Still an underestimate |
| 12 days | about 88% | Close, and useful with caution |
| 2 to 3 weeks | 95% or more | A level that can be acted on |
Reasons a phenobarbital level is not what you expected
| Situation | Effect | What to do |
|---|---|---|
| Sample taken before steady state | Lower than the eventual concentration | Wait two to three weeks after the change, then repeat |
| Patient also taking primidone | A phenobarbital concentration in someone not prescribed phenobarbital | Check the prescription — primidone is metabolised to phenobarbital |
| Long-standing treatment (tolerance) | A high concentration in a patient with no sedation | Treat the patient, not the number; do not reduce a dose that is controlling seizures on the strength of a level alone |
| Enzyme induction by phenobarbital itself | Other drugs — warfarin, oral contraceptives, lamotrigine, direct oral anticoagulants — are cleared faster | Review the whole prescription; phenobarbital is a potent inducer of CYP enzymes and of P-glycoprotein |
| Pregnancy | Concentrations commonly fall through pregnancy | Monitor more often and involve the epilepsy team; withdrawal of treatment carries its own risk |
| Neonates and infants | Half-life is considerably longer at birth and shortens over weeks | Interpret against paediatric guidance, not adult figures |
Four days, a soft range, and a level that never decides the dose on its own
Phenobarbital is reported in micrograms per millilitre — identical to milligrams per litre — or in micromoles per litre. Its molecular weight is 232.24, so one microgram per millilitre is 4.30589 micromoles per litre, and the 20 µg/mL that sits in the middle of most quoted ranges is 86 µmol/L. That is the whole of the arithmetic, and it is the least interesting thing about a phenobarbital level.
The first thing that matters is the half-life. Phenobarbital is eliminated slowly — around 80 to 100 hours in adults, longer in newborns — so the concentration approaches a new steady state over two to three weeks rather than over days. A level taken four days after a dose increase shows roughly half of the rise that increase will eventually produce. The clinical consequence is specific and common: the early level looks acceptable, nobody changes anything, and the patient becomes drowsy a fortnight later as the concentration keeps climbing towards its true plateau. Unless a level is being drawn to check adherence or to investigate suspected toxicity, it is worth waiting the full two to three weeks, and the arithmetic of how far along that curve any given day falls is what the steady-state calculator on this site is for.
The second thing is that the range is soft. The BNF gives 15 to 40 mg/L and the ILAE’s therapeutic drug monitoring position paper gives 10 to 40 mg/L; the upper limits agree and the lower limits do not, which is a fair reflection of how weakly the lower bound is anchored. Tolerance is the reason. Sedation, the effect that limits the dose in someone new to phenobarbital, wanes over weeks to months of continued treatment, so a long-treated patient may be alert and working at a concentration that would have put them to sleep at the start. Plenty of patients are entirely seizure-free below the quoted range, and some need and tolerate concentrations above it. The range therefore describes where most useful concentrations lie in a population; it does not say where this patient’s should be. That is established by their seizure frequency and their side effects over time.
Two practical traps are worth naming. Primidone is metabolised to phenobarbital, so a patient prescribed primidone will have a measurable phenobarbital concentration — check the prescription before puzzling over an unexpected result. And phenobarbital is a powerful inducer of cytochrome P450 enzymes and of P-glycoprotein, so an unexpected level often turns out to be part of a wider interaction story involving warfarin, hormonal contraception, lamotrigine or a direct oral anticoagulant. Whatever the level shows, the decision that follows is a clinical one: an antiseizure medicine is not increased because a number is low, nor reduced or stopped because a number is high, and abrupt withdrawal risks status epilepticus. The level informs the prescriber; it does not replace them.
Frequently asked questions
How do you convert phenobarbital from µg/mL to µmol/L?
Multiply by 4.30589, which is one milligram per litre divided by phenobarbital’s molecular weight of 232.24 g/mol. A level of 20 µg/mL is 86 µmol/L. Divide by the same factor to go back. µg/mL, mcg/mL and mg/L are all the same number, so no conversion is needed between them.
What is the therapeutic range for phenobarbital?
The BNF gives 15–40 mg/L, which is 65–172 µmol/L. The ILAE therapeutic drug monitoring position paper gives 10–40 mg/L (43–172 µmol/L), and Mayo Clinic Laboratories report 10.0–40.0 µg/mL for adults and 15–30 µg/mL for infants and children, with 60 µg/mL as a critical value. The upper limit is agreed; the lower one is not, and the range is a guide rather than a target — many patients are well controlled outside it in both directions.
How long after a dose change should a phenobarbital level be taken?
Two to three weeks. The half-life is about four days in adults, and a concentration needs roughly five half-lives to reach steady state, so a level drawn after four days shows only about half of the eventual rise. Acting on an early level is how patients end up sedated a fortnight later: the number looked acceptable at the time and then kept climbing. Levels taken sooner are still useful for checking adherence or investigating suspected toxicity.
Why can a patient be well with a phenobarbital level above the range?
Because tolerance to the sedative effect develops over weeks to months of continuous treatment. Someone who has taken phenobarbital for years may be fully alert at a concentration that would sedate a person starting the drug, while another patient may be drowsy within the range. That is why the range guides rather than dictates, and why a dose that is controlling seizures is not reduced on the strength of a level alone.
Why does my patient have a phenobarbital level when they are not prescribed it?
Check whether they are taking primidone, which is metabolised to phenobarbital — a measurable phenobarbital concentration is the expected finding in anyone on primidone, and in some laboratories it is reported alongside the primidone level. Other explanations include a recent change of brand or formulation, an unreported over-the-counter or imported product, and simple transcription error, but primidone is by far the commonest.
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References
- Patsalos PN, Berry DJ, Bourgeois BFD, et al. Antiepileptic drugs — best practice guidelines for therapeutic drug monitoring: a position paper by the subcommission on therapeutic drug monitoring, ILAE Commission on Therapeutic Strategies. Epilepsia. 2008;49(7):1239–1276. Phenobarbital reference range 10–40 mg/L.
- Joint Formulary Committee. Phenobarbital — Monitoring requirements. British National Formulary. London: BMJ Group and Pharmaceutical Press. Therapeutic plasma concentration 15–40 mg/L.
- Mayo Clinic Laboratories. Test ID: PBR — Phenobarbital, Serum. Therapeutic range 10.0–40.0 mcg/mL (adults), 15–30 mcg/mL (infants and children); critical value 60.0 mcg/mL and above; half-life approximately 96 hours.
- Patsalos PN, Spencer EP, Berry DJ. Therapeutic drug monitoring of antiepileptic drugs in epilepsy: a 2018 update. Ther Drug Monit. 2018;40(5):526–548.
- World Health Organization. WHO Model List of Essential Medicines. Geneva: WHO. Phenobarbital is listed as an antiseizure medicine, including for neonatal seizures.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.