Barbiturate Anticonvulsant Pregnancy: §4.6: phenobarbital should not be used in women of childbearing potential unless the potential benefit is judged to outweigh the risks after careful consideration of alternatives; consider a pregnancy test before starting, and use highly effective contraception during treatment and for 2 months after the last dose (note that enzyme induction may cause hormonal contraceptive failure — use two complementary methods including a barrier method, a higher-dose oestrogen oral contraceptive, or a non-hormonal intrauterine device). Phenobarbital readily crosses the placenta and is distributed throughout foetal tissue, with maternal and neonatal concentrations similar; animal studies have shown reproductive toxicity in rodents. Monotherapy is preferred in pregnancy where possible, and sudden discontinuation should be avoided. §4.8 lists neonatal sedation, neonatal drug dependence and withdrawal syndrome, neonatal bleeding due to vitamin K deficiency, cleft lip and palate and other congenital malformations. NOTE: the §4.6 text was truncated at the source-fetch limit.

Phenobarbital (Paediatric Epilepsy — Beyond Neonatal)

Brand names: Phenobarbital (generic)

Phenobarbital is a long-acting barbiturate anti-epileptic used in children beyond the neonatal period for generalised and focal seizures, often where newer agents are unsuitable or unavailable.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: 2 to 3 mg/kg per day

Route: Oral (50 mg/5 ml oral suspension; may be administered via nasogastric or PEG feeding tubes, rinsing the tube with 2 ml of water immediately after administration)

Frequency: Once daily at bedtime

Source: UK SPC (eMC) for Phenobarbital Colonis 50 mg/5 ml Oral Suspension, §4.2 (https://www.medicines.org.uk/emc/product/102162/smpc). No maximum dose is stated in §4.2. It may take more than two weeks for the medicine to reach sufficient levels in the bloodstream to control seizures; this also applies when the dose is adjusted. When therapeutic drug monitoring is justified, current clinical practice guidelines for phenobarbital blood level monitoring should be followed. PAEDIATRIC POSOLOGY is captured in the paedDose block (weight-banded per-kg dosing). ADMINISTRATION: shake the bottle well before use; a 10 ml graduated oral syringe (0.2 ml graduations) and press-in bottle adapter are supplied; when giving to a child, ensure the child is sitting up or standing, place the syringe barrel-opening between the gums and the inside of the cheek, and push the plunger slowly to allow swallowing and avoid choking, then give water to wash the medicine down. §4.4: phenobarbital is not effective in absences and myoclonic seizures, which can sometimes be aggravated; prolonged intake (100 mg per day for 3 months) may result in a dependence syndrome; avoid sudden withdrawal to prevent rebound seizures; use with caution in the young, the elderly, in debilitated patients and in those with depressive disorders; monitor for suicidal ideation and behaviour and for serious skin reactions (SJS/TEN/DRESS/AGEP) — discontinue at the first emergence of rash, mucosal lesions or any other manifestation of skin hypersensitivity, and never re-expose after SJS or TEN.

Paediatric dose

Route: Oral

Frequency: As one or two divided doses per day

Max: Not stated in §4.2

SPC §4.2 'Paediatric population (by weight)' gives three weight bands rather than a single per-kg figure, so dosePerKg is left null: less than 20 kg — 5 mg/kg per day, as one or two divided doses; between 20 and 30 kg — 3 to 4 mg/kg per day, as one or two divided doses; above 30 kg — 2 to 3 mg/kg per day, as one or two divided doses. It may take more than two weeks to reach sufficient blood levels to control seizures, including after a dose adjustment. §4.3 contraindicates phenobarbital in hyperkinetic children, and §4.8 notes aggression, hyperactivity and behavioural disturbances particularly in children. Verify all paediatric dosing against a children's formulary before prescribing.

Dose adjustments

Renal

Severe renal impairment is a contraindication (§4.3). §4.4 states the dosage should be reduced in renal insufficiency and hepatic insufficiency (biological monitoring advised) — the §4.4 text was truncated at the source-fetch limit, so confirm the full wording against the SPC.

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC; US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

Hypersensitivity to phenobarbital, other barbiturates or to any of the excipients
Severe respiratory depression
Severe hepatic or renal impairment
Acute intermittent porphyria
Treatment with cobicistat, rilpivirine, telaprevir, cholic acid, delamanid, daclatasvir, dasabuvir, ombitasvir-paritaprevir, ledipasvir, sofosbuvir or voriconazole; and in combination with St John's wort
Hyperkinetic children

Side effects

Sedation — the most frequent adverse effect; also drowsiness, cognitive impairment and memory impairment
Ataxia, dizziness, headache; dyskinesia, nystagmus, lethargy, irritability, grand mal convulsion; behavioural disturbances in children
Psychiatric: abnormal behaviour, agitation, aggression (particularly in children), altered mood, sleep disorders/insomnia, dependence, depression, hallucination, paradoxical excitement, suicidal ideation, withdrawal syndrome, hyperactivity (particularly in children)
Serious skin reactions: allergic dermatitis, fixed pigmented erythema, erythema multiforme, Stevens-Johnson syndrome, toxic epidermal necrolysis, exfoliative dermatitis, DRESS and AGEP; antiepileptic hypersensitivity syndrome
Haematological and metabolic: pancytopenia, aplastic anaemia, agranulocytosis, neutropenia, thrombocytopenia, folate deficiency anaemia, hypocalcaemia, vitamin D and vitamin K abnormalities; also respiratory depression, hypotension, hepatitis/cholestasis, and reduced bone mineral density with osteopenia, osteoporosis, osteomalacia, fractures and rickets
NOTE: the §4.8 table was truncated at the source-fetch limit — this list is incomplete

Interactions

NOTE: the UK §4.5 section was not present in the fetched bundle — the first entry is from UK §4.3/§4.6, the remainder from the US label; source the full UK §4.5 separately
Contraindicated combinations (UK §4.3): cobicistat, rilpivirine, telaprevir, cholic acid, delamanid, daclatasvir, dasabuvir, ombitasvir-paritaprevir, ledipasvir, sofosbuvir, voriconazole, St John's wort
Hormonal contraceptives — due to enzyme induction, phenobarbital may cause failure of the therapeutic effect of oral contraceptives containing oestrogen and/or progesterone; other contraceptive methods should be used (UK §4.6)
Oral anticoagulants (warfarin, acenocoumarol, dicoumarol, phenprocoumon) — barbiturates induce hepatic microsomal enzymes, increasing metabolism and decreasing anticoagulant response; dosage adjustment may be required when phenobarbital is added or withdrawn (US label)
Corticosteroids — barbiturates enhance the metabolism of exogenous corticosteroids; dosage adjustment may be required when phenobarbital is added or withdrawn (US label)
Griseofulvin — phenobarbital appears to interfere with absorption of orally administered griseofulvin, decreasing its blood level (US label)

Clinical monograph

How it works

It potentiates GABA-mediated inhibition by prolonging chloride channel opening at the GABA-A receptor, raising the seizure threshold and reducing neuronal excitability.

Prescribing in practice

Marked sedation, behavioural disturbance and cognitive/learning impairment are common in children and must be weighed against benefit, with abrupt withdrawal avoided because it can precipitate status epilepticus.
It is a potent hepatic enzyme inducer that lowers levels of many co-administered drugs, including other anti-epileptics, corticosteroids and hormonal contraception in adolescents.
Doses are weight-based and require titration; check against a children's formulary as requirements change with growth.

Monitoring

Monitor seizure frequency, sedation and behaviour, with plasma-level measurement reserved for suspected toxicity, poor control or adherence concerns.

Counselling the patient

Do not stop the medicine suddenly as this can trigger seizures.
Report excessive drowsiness, mood or behaviour changes, or a rash.
Tell other prescribers about this medicine because it can reduce the effect of many other drugs.

Evidence & guidelines

Phenobarbital is a long-established anti-epileptic recognised by the World Health Organization as an essential medicine, though sedation limits first-line use where alternatives exist.

Reference: NICE NG217 Epilepsy; PALS Advanced Life Support Manual; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

Calculators

Pathways