Phenobarbital
phenobarbital — barbiturate anticonvulsant (5-ethyl-5-phenylbarbituric acid)
Indications
| Indication | Approved Population | Therapy Type | Status |
|---|---|---|---|
| Generalized tonic-clonic (grand mal) seizures | Adults and children | Monotherapy or adjunctive | FDA Approved |
| Partial (focal) seizures | Adults and children | Monotherapy or adjunctive | FDA Approved |
| Status epilepticus | Adults and children (IV formulation) | Second/third-line after benzodiazepines | FDA Approved |
| Sedation | Adults | Short-term only | FDA Approved |
Phenobarbital is one of the oldest antiseizure medications still in clinical use, first introduced in 1912 under the brand name Luminal. It remains on the WHO List of Essential Medicines and is recommended by the WHO as a first-line anticonvulsant in resource-limited settings due to its low cost, wide availability, and once-daily dosing. In high-income countries, phenobarbital has largely been supplanted by newer agents for most adults due to its adverse cognitive and sedative profile, but it retains important roles in neonatal seizures (where it remains first-line), status epilepticus (as second/third-line after benzodiazepines), and in settings where cost is a critical barrier. Phenobarbital is effective against all seizure types except absence seizures.
Neonatal seizures: First-line treatment worldwide; loading dose of 15–20 mg/kg IV is standard practice. Not formally listed in the FDA label but is universally accepted as the first-line neonatal anticonvulsant. Evidence quality: Moderate (based on decades of clinical experience and the NEOLEV2 trial comparing with levetiracetam).
Alcohol withdrawal: Used as an adjunct or alternative to benzodiazepines in severe or refractory alcohol withdrawal syndrome. Evidence quality: Moderate.
Neonatal hyperbilirubinemia: Low doses induce glucuronyl transferase activity to enhance bilirubin conjugation (Crigler-Najjar type II, Gilbert syndrome). Evidence quality: Low.
Dosing
Dosing by Clinical Scenario
| Clinical Scenario | Starting Dose | Maintenance Dose | Maximum Dose | Notes |
|---|---|---|---|---|
| Adult epilepsy — chronic maintenance | 60 mg/day (or 1–3 mg/kg/day) | 60–200 mg/day | ~300 mg/day (guided by TDM) | Once daily at bedtime preferred; start at 25% of target and increase weekly to minimize sedation Target serum level 10–40 mcg/mL; steady state in 2–4 weeks |
| Pediatric epilepsy — chronic maintenance | 3–6 mg/kg/day | 3–6 mg/kg/day in 1–2 divided doses | Guided by serum levels | Higher per-kg doses needed in children due to faster clearance; once or twice daily dosing Children clear phenobarbital faster than adults (t½ 30–70 h) |
| Neonatal seizures — acute loading | 15–20 mg/kg IV over 10–15 min | 3–5 mg/kg/day in 1–2 divided doses | Additional 5–10 mg/kg boluses up to total 40 mg/kg if needed | First-line for neonatal seizures worldwide; loading produces levels ~20 mcg/mL Neonatal t½ very long (60–180 h); monitor closely for respiratory depression |
| Status epilepticus — IV loading (adults) | 10–20 mg/kg IV | Per clinical response and TDM | Rate ≤60 mg/min; total guided by response | Second/third-line after benzodiazepines and phenytoin/fosphenytoin; have resuscitation equipment available Monitor respiratory status, BP, and level of consciousness throughout infusion |
| Elderly or debilitated patients | Lowest effective dose | Reduce by 25–50% vs standard | Guided by TDM | Increased sensitivity; may react with excitement, depression, or confusion Decreased clearance with aging; lower doses needed |
| Renal or hepatic impairment | Reduce dose | Guided by TDM | Guided by TDM | 25–50% excreted unchanged renally; hepatic metabolism is primary route Contraindicated in marked hepatic impairment or respiratory disease with obstruction |
Because of phenobarbital's very long half-life (mean ~79 hours in adults), steady-state concentrations are not achieved for 2–4 weeks after initiating or changing a dose. Without a loading dose, it will take several weeks to reach therapeutic levels. To minimize initial sedation, start at approximately 25% of the target dose and increase by 25% each week. When rapid therapeutic levels are needed (e.g., status epilepticus), an IV loading dose of 15–20 mg/kg is given, producing immediate therapeutic concentrations. Weekly serum levels during initiation are recommended to prevent overshoot.
Phenobarbital is a narrow-therapeutic-index drug. The therapeutic range of 10–40 mcg/mL provides anticonvulsant benefit in most patients, but ataxia and nystagmus may appear above 40 mcg/mL, severe toxicity above 60 mcg/mL, and levels exceeding 80–100 mcg/mL are potentially lethal. All dose adjustments should be guided by serum levels, not empiric dose escalation alone. At steady state, dose changes produce proportional changes in serum levels (linear pharmacokinetics), simplifying TDM-guided adjustments.
Pharmacology
Mechanism of Action
Phenobarbital exerts its anticonvulsant effect primarily by enhancing GABA-A receptor-mediated inhibitory neurotransmission. Unlike benzodiazepines, which increase the frequency of chloride channel opening, phenobarbital prolongs the duration of chloride channel opening at GABA-A receptors, producing more sustained neuronal hyperpolarization and raising the seizure threshold. At higher (supratherapeutic) concentrations, phenobarbital also directly activates the chloride channel independently of GABA and inhibits excitatory glutamatergic neurotransmission by blocking AMPA and kainate receptor subtypes. This dual mechanism — enhancing inhibition while suppressing excitation — gives phenobarbital a broad-spectrum antiseizure profile against all seizure types except absence seizures, where it may paradoxically worsen seizure control.
ADME Profile
| Parameter | Value | Clinical Implication |
|---|---|---|
| Absorption | Oral bioavailability ~95%; Tmax 2–8 h (oral); peak brain concentrations 10–15 h; absorption slow but nearly complete; enhanced when taken on empty stomach | Long time-to-peak brain levels means oral dosing is not suitable for acute seizure control; IV loading is required for emergencies (onset ~5 min IV) |
| Distribution | Vd 0.5–0.6 L/kg; protein binding 40–60% (primarily albumin); lowest lipid solubility among barbiturates | Low lipid solubility means slowest CNS penetration of all barbiturates but longest duration of action; distributes to all tissues including fetal brain and placenta |
| Metabolism | Hepatic microsomal enzymes (primarily CYP2C9); potent inducer of CYP1A2, CYP2B6, CYP2C8, CYP2C9, CYP3A4, and P-glycoprotein; no active metabolites; metabolites excreted as glucuronic acid conjugates | Enzyme induction is the defining interaction characteristic; accelerates metabolism of numerous co-administered drugs including oral contraceptives, warfarin, corticosteroids, and other ASMs |
| Elimination | t½ 53–118 h adults (mean 79 h); 60–180 h neonates; 30–70 h children; 25–50% excreted unchanged in urine; renal excretion is pH-dependent (increased at alkaline pH) | Very long half-life supports once-daily dosing but means 2–4 weeks to steady state; urinary alkalinization can enhance elimination in overdose; first-order kinetics allows linear dose-level predictions |
Side Effects
Phenobarbital was approved decades before modern standardized adverse-event reporting. Incidence figures below are derived from post-marketing surveillance data compiled across thousands of hospitalized patients, the FDA-approved labeling, and published clinical experience. Most adverse effects are dose-dependent and correlated with serum levels. Unlike newer ASMs, phenobarbital lacks placebo-controlled incidence data from registration trials.
| Adverse Effect | Incidence | Clinical Note |
|---|---|---|
| Sedation / drowsiness | Common early; tolerance develops within 2–3 weeks | The most clinically significant effect; dose-dependent; correlates with serum level; tolerance develops with chronic use but cognitive impairment may persist |
| Cognitive impairment | Significant in long-term use | Decreased attention, processing speed, memory; associated with cognitive deficits in children treated for febrile seizures; most limiting factor for long-term use in developed countries |
| Adverse Effect | Incidence | Clinical Note |
|---|---|---|
| Somnolence (as reported event) | 1–3% (surveillance data) | Most common formally reported adverse reaction; generally dose-related |
| Skin rash | 1–3% | Usually mild maculopapular, morbilliform, or scarlatiniform; resolves on discontinuation |
| Ataxia / dizziness | Dose-related (levels >40 mcg/mL) | Sign of toxicity; warrants level check and dose reduction |
| Nausea / vomiting | <1–3% | GI effects usually mild and transient |
| Paradoxical excitement (children) | Common in children | Hyperactivity, irritability, aggression instead of sedation; sometimes necessitates drug change |
| Confusion / depression (elderly) | Common in elderly | Elderly patients frequently react with excitement, confusion, or depression; start with lower doses |
| Adverse Effect | Estimated Frequency | Typical Onset | Required Action |
|---|---|---|---|
| Stevens-Johnson Syndrome / Toxic Epidermal Necrolysis | Rare | Weeks to months | Immediate discontinuation; potentially fatal; dermatology consultation; never rechallenge |
| Respiratory depression / apnea | Dose-dependent; primarily with IV loading | Minutes (IV); hours (oral overdose) | Resuscitation equipment must be available for IV administration; mechanical ventilation may be required; more common in neonates and with CNS depressant co-administration |
| Physical dependence and withdrawal seizures | Expected with prolonged use at high doses | 12–72 hours after abrupt discontinuation | Never stop abruptly; gradual taper over weeks to months; abrupt withdrawal can cause delirium, seizures, status epilepticus, and death |
| Megaloblastic anemia (folate deficiency) | Uncommon; chronic use | Months to years | Monitor CBC periodically; supplement folic acid (1 mg/day); check serum folate and B12 if macrocytosis develops |
| Osteomalacia / vitamin D deficiency | Significant risk with chronic use | Months to years | CYP induction accelerates vitamin D metabolism; supplement vitamin D and calcium; monitor bone density in chronic users; higher fracture risk |
| Connective tissue disorders (Dupuytren contracture, frozen shoulder) | Uncommon; chronic use | Years of use | Monitor for shoulder-hand syndrome; consider alternative ASM if symptomatic |
| Hepatotoxicity | Rare | Variable | Monitor LFTs periodically; hepatic carcinoma seen in animal models (phenobarbital is carcinogenic in mice and rats) |
| Suicidal behavior and ideation | AED class effect (RR 1.8 vs placebo) | As early as 1 week | AED class warning; monitor for depression, mood changes, suicidal ideation |
| Fetal harm (Pregnancy Category D) | Increased incidence of fetal abnormalities | Throughout pregnancy | Barbiturates cross placenta; neonatal withdrawal syndrome reported; counsel regarding risks; supplement folate and vitamin K in pregnancy |
Sedation and cognitive impairment are the primary reasons phenobarbital has been supplanted by newer ASMs in high-income countries. Tolerance to sedation typically develops within 2–3 weeks of stable dosing, but measurable cognitive slowing may persist even after tolerance to obvious drowsiness develops. In children, the drug is associated with cognitive deficits even at therapeutic doses, which contributed to the decline of its use for febrile seizure prophylaxis. Bedtime dosing, slow titration, and use of the lowest effective dose all help mitigate the sedation burden.
Drug Interactions
Phenobarbital is one of the most potent hepatic enzyme inducers in clinical use, accelerating the metabolism of a vast number of co-administered drugs through induction of multiple CYP isoenzymes (CYP1A2, CYP2B6, CYP2C8, CYP2C9, CYP3A4) and P-glycoprotein. This enzyme-inducing property is the single most important pharmacological consideration when prescribing phenobarbital, as it can render many other medications therapeutically ineffective at standard doses.
When phenobarbital is discontinued in a patient on multiple medications, the removal of enzyme induction causes a gradual rise in levels of all co-administered drugs that were being induced. This effect takes 2–4 weeks to fully manifest (reflecting the time to enzyme de-induction). For drugs with narrow therapeutic windows — particularly warfarin, phenytoin, and immunosuppressants — failure to pre-emptively reduce doses during phenobarbital taper can cause serious toxicity, including bleeding or drug-level-dependent adverse effects.
Monitoring
-
Serum Phenobarbital Level
Weekly during initiation; trough level at steady state; after dose changes
Routine Therapeutic range 10–40 mcg/mL. Ataxia/nystagmus typically above 40 mcg/mL; severe toxicity above 60 mcg/mL; potentially lethal above 80–100 mcg/mL. Draw trough level. Steady state requires 2–4 weeks. Linear pharmacokinetics allows proportional dose-level adjustments. -
Complete Blood Count
Baseline, then periodically (annually)
Routine Monitor for megaloblastic anemia (folate deficiency) with chronic use. Supplement folic acid 1 mg/day, especially in pregnancy. -
Hepatic Function (LFTs)
Baseline, then periodically
Routine Phenobarbital is hepatically metabolized and contraindicated in marked hepatic impairment. Liver damage reported rarely. Monitor for signs of hepatic coma (avoid use if premonitory signs present). -
Vitamin D and Bone Density
Annually in chronic users
Routine CYP induction accelerates vitamin D catabolism, leading to osteomalacia and increased fracture risk with long-term use. Supplement vitamin D (1,000–2,000 IU/day) and calcium. Consider DEXA scan in patients on chronic therapy. -
Renal Function
Baseline, then periodically
Routine 25–50% of phenobarbital is eliminated unchanged in urine. Impaired renal function can prolong drug effect. Dose reduction may be needed. -
Mood and Suicidality
Every visit, ongoing
Routine AED class warning; monitor for depression, mood changes, behavioral disturbance, and suicidal ideation at each visit. Phenobarbital itself can cause depression as an adverse effect. -
Concomitant Drug Levels
When adding, adjusting, or stopping phenobarbital
Trigger-based Due to potent enzyme induction, monitor levels of co-administered drugs (warfarin INR, phenytoin, lamotrigine, cyclosporine, etc.) when starting or stopping phenobarbital. Allow 2–4 weeks for full induction/de-induction. -
Respiratory Status (IV administration)
Continuously during IV loading
Trigger-based Respiratory depression is the primary acute risk with IV phenobarbital. Monitor oxygen saturation, respiratory rate, and level of consciousness continuously during and after IV loading. Have intubation equipment available.
Contraindications & Cautions
Absolute Contraindications
- Hypersensitivity to barbiturates
- History of manifest or latent porphyria — barbiturates can precipitate acute porphyric crises by inducing ALA synthase
- Marked hepatic impairment — phenobarbital is hepatically metabolized; risk of accumulation and hepatic coma
- Respiratory disease with dyspnea or obstruction — barbiturates are respiratory depressants; contraindicated when respiratory function is compromised
Relative Contraindications (Specialist Input Recommended)
- History of drug addiction — phenobarbital has abuse potential (Schedule IV); physical dependence develops with chronic use
- Severe renal impairment — 25–50% eliminated unchanged renally; accumulation risk
- Pregnancy — Category D; associated with increased fetal abnormalities, neonatal withdrawal, and hemorrhagic disease of the newborn; supplement vitamin K peripartum
- Patients with depression or suicidal tendencies — barbiturates can worsen depression; AED class warning also applies
Use with Caution
- Elderly or debilitated patients — may react with paradoxical excitement, confusion, or depression; start with reduced doses
- Children — may produce paradoxical excitement and hyperactivity; cognitive deficits documented with chronic use
- Mild to moderate hepatic impairment — administer with caution and initially in reduced doses
- Acute or chronic pain — may cause paradoxical excitement or mask important symptoms
- Patients on concomitant CNS depressants — additive CNS depression; risk of respiratory failure and death
- Patients on corticosteroid therapy or with borderline hypoadrenal function — enzyme induction may diminish corticosteroid effects
Antiepileptic drugs, including phenobarbital, increase the risk of suicidal thoughts or behavior. Pooled analysis of 199 placebo-controlled trials of 11 AEDs showed approximately twice the risk (adjusted RR 1.8; 95% CI: 1.2–2.7) in AED-treated patients compared to placebo. Monitor all patients for emergence or worsening of depression, suicidal thoughts, or unusual changes in behavior.
Patient Counselling
Purpose of Therapy
Phenobarbital is one of the oldest and most effective medications for preventing seizures. It works by calming excess electrical activity in the brain. While it has been used for over 100 years, it requires careful blood level monitoring and carries risks of physical dependence if stopped suddenly. Your doctor will regularly check blood tests to make sure the drug level is in the right range.
How to Take
Take phenobarbital at the same time each day, typically at bedtime to minimize daytime drowsiness. It may be taken with or without food, though absorption is slightly faster on an empty stomach. The dose should never be changed or stopped without consulting your doctor. If starting the medication, drowsiness will likely improve within 2–3 weeks as your body adjusts.
Sources
- Phenobarbital Tablets, USP. Full Prescribing Information. Westminster Pharmaceuticals, LLC. Drugs.com PI Primary US prescribing information source for phenobarbital tablets, including dosing, contraindications, warnings, and drug interactions.
- Phenobarbital Sodium Injection. DailyMed Label. Hikma Pharmaceuticals USA Inc. DailyMed Injectable formulation labeling with IV/IM dosing guidelines for status epilepticus and acute seizure management.
- Brodie MJ, Kwan P. Current position of phenobarbital in epilepsy and its future. Epilepsia. 2012;53(Suppl 8):40–46. doi:10.1111/epi.12027 Comprehensive review of phenobarbital's current clinical role, comparing efficacy to newer agents and discussing its global importance.
- Kwan P, Brodie MJ. Phenobarbital for the treatment of epilepsy in the 21st century: a critical review. Epilepsia. 2004;45(9):1141–1149. doi:10.1111/j.0013-9580.2004.12704.x Critical analysis of phenobarbital's efficacy evidence base, adverse effect profile, and positioning relative to newer antiseizure medications.
- Sharpe C, Reiner GE, Davis SL, et al. Levetiracetam versus phenobarbital for neonatal seizures: a randomized controlled trial. Pediatrics. 2020;145(6):e20193182. doi:10.1542/peds.2019-3182 NEOLEV2 trial: phenobarbital was more effective than levetiracetam for neonatal seizures (80% vs 28% response), reinforcing its first-line role.
- Mattson RH, Cramer JA, Collins JF, et al. Comparison of carbamazepine, phenobarbital, phenytoin, and primidone in partial and secondarily generalized tonic-clonic seizures. N Engl J Med. 1985;313(3):145–151. doi:10.1056/NEJM198507183130303 Landmark VA Cooperative Study comparing four first-generation ASMs; phenobarbital had equivalent efficacy but higher behavioral adverse effects.
- World Health Organization. WHO Model List of Essential Medicines — 23rd List, 2023. Geneva: WHO; 2023. WHO EML Phenobarbital is listed as a WHO Essential Medicine for epilepsy management, supporting its role as the primary ASM in resource-limited settings.
- Glauser T, Shinnar S, Gloss D, et al. Evidence-based guideline: treatment of convulsive status epilepticus in children and adults. Report of the Guideline Committee of the American Epilepsy Society. Epilepsy Curr. 2016;16(1):48–61. doi:10.5698/1535-7597-16.1.48 AES guideline for status epilepticus positioning phenobarbital as a second/third-line option after benzodiazepines.
- Rogawski MA, Loscher W. The neurobiology of antiepileptic drugs. Nat Rev Neurosci. 2004;5(7):553–564. doi:10.1038/nrn1430 Authoritative review of antiepileptic drug mechanisms including the distinction between phenobarbital's GABA-A channel duration prolongation and benzodiazepine frequency-enhancing action.
- Phenobarbital. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024. StatPearls Comprehensive clinical pharmacology reference covering mechanism of action, CYP induction, contraindications, and toxicity management.
- Nelson E, Powell JR, Conrad K, et al. Phenobarbital pharmacokinetics and bioavailability in adults. J Clin Pharmacol. 1982;22(2-3):141–148. doi:10.1002/j.1552-4604.1982.tb02662.x Definitive adult pharmacokinetic study establishing 95% bioavailability, Vd 0.6 L/kg, and mean elimination half-life of 5.1–5.8 days.
- Painter MJ, Scher MS, Stein AD, et al. Phenobarbital compared with phenytoin for the treatment of neonatal seizures. N Engl J Med. 1999;341(7):485–489. doi:10.1056/NEJM199908123410704 Comparative trial of phenobarbital vs phenytoin for neonatal seizures showing similar efficacy; established standard neonatal loading dose of 20 mg/kg.
- Pack AM, Morrell MJ, Randall A, McMahon DJ, Shane E. Bone health in young women with epilepsy after one year of antiepileptic drug monotherapy. Neurology. 2008;70(18):1586–1593. doi:10.1212/01.wnl.0000310981.44780.a0 Demonstrates bone density effects of enzyme-inducing ASMs including phenobarbital through accelerated vitamin D catabolism.
- Farwell JR, Lee YJ, Hirtz DG, Sulzbacher SI, Ellenberg JH, Nelson KB. Phenobarbital for febrile seizures — effects on intelligence and on seizure recurrence. N Engl J Med. 1990;322(6):364–369. doi:10.1056/NEJM199002083220604 Key study demonstrating cognitive deficits (lower IQ scores) in children receiving phenobarbital for febrile seizure prophylaxis, contributing to the decline of this indication.