Phenobarbital


Generic Medicine Info
Indications and Dosage
Intramuscular, Intravenous
Sedation
Adult: 30-120 mg daily in 2-3 divided doses via deep IM inj or slow IV inj at a rate of no more than 60 mg/min. Max: 400 mg daily.
Elderly: Dose reduction may be required.

Intramuscular
Preoperative sedation
Adult: 100-200 mg via deep inj, to be given 60-90 minutes before surgery.
Elderly: Dose reduction may be required.

Intravenous
Status epilepticus
Adult: 10 mg/kg (Max: 1,000 mg for each dose) given at a rate of not more than 100 mg/min. Dosage and treatment recommendations may vary among countries and individual products (refer to specific product guidelines).
Child: 15-20 mg/kg via slow inj; may repeat with additional 5-10 mg/kg after 10 minutes. Dosage recommendations may vary among countries and individual products (refer to specific product guidelines).

Oral
Sedation
Adult: 30-120 mg daily in 2-3 divided doses. Max: 400 mg daily.
Elderly: Dose reduction may be required.

Oral
Generalised tonic-clonic seizures, Partial seizures
Adult: 60-180 mg once daily at night. Alternatively, doses up to 350 mg daily in divided doses may be given. Adjust dose according to individual needs to achieve adequate seizure control. Dosage recommendations may vary among countries and individual products (refer to specific product guidelines).
Child: 5-8 mg/kg daily. Alternatively, 3-6 mg/kg daily. Dosage recommendations may vary among countries and individual products (refer to specific product guidelines).
Elderly: Dose reduction may be required.
Special Patient Group
Debilitated patients: Dose reduction may be required.
Renal Impairment
Dose reduction may be required. Severe: Contraindicated.
Hepatic Impairment
Dose reduction may be required. Severe: Contraindicated.
Administration
May be taken with or without food.
Contraindications
Acute intermittent porphyria or history of manifest or latent porphyria; severe respiratory depression. IV: History of sedative-hypnotic substance use disorder; nephritic patients (large doses). Severe renal and hepatic impairment.
Special Precautions
Patient with respiratory disease (including status asthmaticus), acute or chronic pain, history of drug abuse or alcoholism, severe anaemia (use is not recommended if due to folate deficiency) or history of haematological disease (particularly chronic anaemia), cardiac disease, hypotension or shock, depression or suicidal tendencies, diabetes mellitus, hyperthyroidism, hypoadrenalism, fever. Avoid abrupt withdrawal. Not indicated for treatment of absence seizures. Consider supplementation of vitamin D and Ca as necessary. Debilitated patients. Mild to moderate renal and hepatic impairment. Children and elderly. Pregnancy and lactation.
Adverse Reactions
Significant: Respiratory depression (particularly IV use), suicidal ideation and behaviour; paradoxical responses (including agitation, hyperactivity); drug dependence, decreased BMD, increased risk of fractures (prolonged use).
Blood and lymphatic system disorders: Agranulocytosis, megaloblastic anaemia, thrombocytopenia.
Cardiac disorders: Bradycardia.
Gastrointestinal disorders: Nausea, vomiting.
General disorders and administration site conditions: Lethargy, hangover effect; inj site reactions (IV/IM).
Hepatobiliary disorders: Hepatitis, cholestasis.
Musculoskeletal and connective tissue disorders: Dupuytren's contracture, arthralgia, frozen shoulder, osteomalacia, rickets; osteopenia, osteoporosis (prolonged use).
Nervous system disorders: Drowsiness, ataxia, nystagmus.
Psychiatric disorders: Mental depression, hallucination; confusion, restlessness (in elderly); memory and cognitive impairment; behavioural disturbances (in children).
Skin and subcutaneous tissue disorders: Maculopapular, morbilliform or scarlatiniform rashes.
Vascular disorders: Hypotension, syncope.
Potentially Fatal: Rarely, Stevens-Johnson syndrome, toxic epidermal necrolysis, exfoliative dermatitis.
IM/IV/Parenteral/PO: D
Patient Counseling Information
This drug may cause drowsiness or reduce alertness, if affected, do not drive or operate machinery.
Monitoring Parameters
Monitor serum phenobarbital concentration (if clinically indicated); LFTs, CBC with differential, and renal function (periodically). Observe for signs and symptoms of suicidality or unusual changes in behaviour. Assess for history of addiction or suicidal ideation; CNS status and seizure activity. For IV inj: Monitor blood pressure, heart and respiratory rate, and administration site.
Overdosage
Symptoms: Ataxia, nystagmus, disinhibition, drowsiness, dysarthria, hypotension, hypotonia, hyporeflexia, hypothermia, erythematous or haemorrhagic blisters, decreased urine formation, pulmonary oedema, CV collapse, cardiac arrest, respiratory depression, coma. Management: Symptomatic and supportive treatment. Maintain adequate airway, provide assisted respiration and oxygen if needed. Monitor and maintain vital signs, blood gases, and serum electrolytes. May administer activated charcoal (adult: 50 g; child under 5 years: 10-15 g) if more than 10 mg/kg has been ingested within 1 hour, provided that airway can be protected; doses may be repeated. Administer dopamine or dobutamine for severe hypotension. Initiate urinary alkalinisation; may consider forced diuresis if renal function is normal. Haemodialysis or peritoneal dialysis may be considered in severe intoxication and/or in patients in shock or are anuric; charcoal haemoperfusion is usually given for severe poisoning that fails to improve or deteriorates despite good supportive care.
Drug Interactions
Concurrent use with MAOIs, SSRIs, and TCAs may antagonise antiepileptic activity of phenobarbital by reducing the convulsive threshold. May result in additive CNS depressant effects when used concomitantly with other CNS depressants (e.g. antihistamines, narcotics, tranquilisers). Increased plasma concentration with oxcarbazepine, phenytoin, methylphenidate, chloramphenicol, valproic acid or Na valproate. May decrease plasma concentration with vigabatrin or folic acid. May decrease efficacy with memantine. May reduce the plasma concentrations of disopyramide, quinidine, chloramphenicol, doxycycline, metronidazole, rifampicin, anticoagulants (e.g. dicoumarol), chlorpromazine, paroxetine, mianserin, TCAs, carbamazepine, lamotrigine, tiagabine, zonisamide, primidone, ethosuximide, antifungals (e.g. itraconazole, posaconazole, griseofulvin, voriconazole), aripiprazole, antivirals (e.g. abacavir, amprenavir, darunavir, lopinavir, indinavir, nelfinavir, saquinavir), clonazepam, aprepitant, β-blockers (e.g. metoprolol, timolol), Ca channel blockers (e.g. felodipine, diltiazem, verapamil, nifedipine), digoxin, ciclosporin, tacrolimus, corticosteroids, etoposide, irinotecan, eplerenone, haloperidol, gestrinone, toremifene, methadone, montelukast, theophylline, sodium oxybate, thyroid hormones, tibolone, tropisetron, vitamin D. May reduce the effect of oral contraceptives containing estrogen and/or progestogen. May increase the metabolism of paracetamol which may lead to reduced effect and increased risk of hepatotoxicity.
Food Interaction
May result in additive CNS depressant effect with alcohol; avoid concomitant use. St. John's wort may decrease the serum phenobarbital levels leading to reduced efficacy.
Lab Interference
May interfere with metyrapone test, phentolamine test, and serum bilirubin estimation.
Action
Description: Phenobarbital is a long-acting barbiturate that has hypnotic, sedative, and anticonvulsant activities. The exact mechanism of action is unknown, but it may be related to its ability to enhance and/or mimic the synaptic action of GABA. It depresses the sensory cortex, reduces motor activity, changes cerebellar function, and produces drowsiness, sedation, and hypnosis. Its anticonvulsant property is exhibited at high doses.
Synonym: phenobarbitone.
Onset: ≥60 minutes (oral); 5 minutes (IV). Peak effect: CNS depression: ≥15 minutes (IV).
Duration: 10-12 hours (oral); >6 hours (IV).
Pharmacokinetics:
Absorption: Readily and completely absorbed from the gastrointestinal tract. Bioavailability: 90% (oral). Time to peak plasma concentration: 2-4 hours (oral).
Distribution: Crosses the placenta and enters breast milk. Volume of distribution: 0.61 L/kg. Plasma protein binding: Approx 45-60%.
Metabolism: Metabolised partly in the liver via oxidation by CYP2C9 and to a lesser extent by CYP2C19 and CYP2E1, and via N-glucosidation.
Excretion: Via urine (25-50% as an unchanged drug); faeces (less common). Elimination half-life: Approx 75-120 hours.
Chemical Structure

Chemical Structure Image
Phenobarbital

Source: National Center for Biotechnology Information. PubChem Database. Phenobarbital, CID=4763, https://pubchem.ncbi.nlm.nih.gov/compound/Phenobarbital (accessed on Jan. 22, 2020)

Storage
Store between 20-25°C. Protect from light. Storage recommendations may vary among individual products. Refer to specific product guidelines.
MIMS Class
Anticonvulsants / Hypnotics & Sedatives
ATC Classification
N03AA02 - phenobarbital ; Belongs to the class of barbiturates and derivatives antiepileptics.
References
Anon. Phenobarbital. AHFS Clinical Drug Information [online]. Bethesda, MD. American Society of Health-System Pharmacists, Inc. https://www.ahfscdi.com. Accessed 27/07/2021.

Anon. Phenobarbital. Lexicomp Online. Hudson, Ohio. Wolters Kluwer Clinical Drug Information, Inc. https://online.lexi.com. Accessed 03/09/2021.

Buckingham R (ed). Phenobarbital. Martindale: The Complete Drug Reference [online]. London. Pharmaceutical Press. https://www.medicinescomplete.com. Accessed 27/07/2021.

Idaman Pharma Phenobarbitone Tablet (Idaman Pharma Manufacturing Sdn. Bhd.). National Pharmaceutical Regulatory Agency - Ministry of Health Malaysia. https://www.npra.gov.my. Accessed 27/07/2021.

Joint Formulary Committee. Phenobarbital. British National Formulary [online]. London. BMJ Group and Pharmaceutical Press. https://www.medicinescomplete.com. Accessed 27/07/2021.

Phenobarbital Accord 15 mg Tablets (Accord Healthcare Limited). MHRA. https://products.mhra.gov.uk. Accessed 27/07/2021.

Phenobarbital Elixir (Westminster Pharmaceuticals). DailyMed. Source: U.S. National Library of Medicine. https://dailymed.nlm.nih.gov/dailymed. Accessed 27/07/2021.

Phenobarbital Oral Solution (Pharmaceutical Associates, Inc.). DailyMed. Source: U.S. National Library of Medicine. https://dailymed.nlm.nih.gov/dailymed. Accessed 27/07/2021.

Phenobarbital Sodium 200 mg/mL Injection (Macarthys Laboratories Ltd). MHRA. https://products.mhra.gov.uk. Accessed 27/07/2021.

Phenobarbital Sodium 30 mg/mL Injection (Macarthys Laboratories Ltd). MHRA. https://products.mhra.gov.uk. Accessed 27/07/2021.

Phenobarbital Sodium Injection (Cameron Pharmaceuticals LLC). DailyMed. Source: U.S. National Library of Medicine. https://dailymed.nlm.nih.gov/dailymed. Accessed 27/07/2021.

Phenobarbital Tablet (e5 Pharma, LLC). DailyMed. Source: U.S. National Library of Medicine. https://dailymed.nlm.nih.gov/dailymed. Accessed 27/07/2021.

PSM Healthcare Limited, t/a API Consumer Brands. Phenobarbitone Tablet 15 mg, 30 mg data sheet April 2017. Medsafe. http://www.medsafe.govt.nz. Accessed 27/07/2021.

Disclaimer: This information is independently developed by MIMS based on Phenobarbital from various references and is provided for your reference only. Therapeutic uses, prescribing information and product availability may vary between countries. Please refer to MIMS Product Monographs for specific and locally approved prescribing information. Although great effort has been made to ensure content accuracy, MIMS shall not be held responsible or liable for any claims or damages arising from the use or misuse of the information contained herein, its contents or omissions, or otherwise. Copyright © 2021 MIMS. All rights reserved. Powered by MIMS.com
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