Phenobarbitone

Class: anticonvulsant – barbiturate

Indications (NB some may be unlicensed): seizure control, status epilepticus, pre-op anxiety, terminal restlessness

Contraindications/cautions: acute intermittent porphyria, severe respiratory depression or pulmonary insufficiency, elderly, renal/hepatic failure

Adverse reactions: common: drowsiness, headache, hypotension, respiratory depression; less common: GI upset, paradoxical excitement, pain, hypocalcaemia,hallucinations

Metabolism/clearance: may be metabolised by metabolising enzyme CYP2C19 mainly in the liver

Interactions:

• increased clinical effect/toxicity of phenobarbitone (due to increased blood concentrations) may occur with some CYP metabolising enzyme inhibitors (see above) e.g. fluconazole, fluoxetine, ketoconazole
• decreased clinical effect/toxicity of phenobarbitone (due to decreased blood concentrations) may occur with some CYP metabolism enzyme inducers (see above) e.g. phenytoin, rifampicin
• decreased clinical effect/toxicity of some drugs (due to decreased blood concentrations of them) may occur with phenobarbitone due to metabolising enzyme induction by phenobarbitone e.g. aprepitant, buspirone, carbamazepine, clonazepam, dexamethasone, diazepam, domperidone, fentanyl, itraconazole, ketoconazole, methadone, midazolam, NSAIDs (e.g. diclofenac), phenytoin, prednisone, quetiapine, triazolam, warfarin
• additive CNS effects with other CNS depressants e.g. benzodiazepines (e.g. lorazepam), phenothiazines (e.g. chlorpromazine), opioids, alcohol
• Refer to PCF8 or NZF for full interactions

Syringe driver: give alone and watch for irritation at injection site

Mechanism of action: depresses activity of all excitable tissue perhaps via GABA

Notes:

• risk of respiratory depression in overdose