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Phenobarbital is a long-acting barbiturate still widely used as an anticonvulsant (seizure medication) and, historically, as a sedative. Its long duration makes it useful for steady seizure control and for managing withdrawal from other depressants. It shares the barbiturate hazards; a narrow safety margin and dangerous synergy with other central nervous system depressants.
- Effective seizure control
- Steady long-acting sedation
- Used in depressant withdrawal management
- Sedation and cognitive dulling
- Respiratory depression in overdose
- Many drug interactions from enzyme induction
- Dependence and withdrawal
Mechanism
Phenobarbital is a positive modulator at the -A receptor that prolongs the opening of the chloride channel in response to GABA (the brain's main inhibitory neurotransmitter), and at higher concentrations can directly activate the channel. It also weakly reduces excitatory signaling. This combination raises the seizure threshold and calms neural firing, which is why it works as an anticonvulsant and sedative. It's also a strong inducer of liver enzymes, which drives many drug interactions.
receptor fingerprint
-A receptor (barbiturate site)Positive allosteric modulator; direct agonist at high doses
Safetyrisks and cautions, not medical advice
Overdose causes profound respiratory depression, and because barbiturates lack a ceiling on their depressant effect the margin for error is small; there's no naloxone-style antidote. It's especially dangerous with alcohol, opioids, or benzodiazepines. It strongly induces liver enzymes, reducing the levels of many other medications, and long-term use causes dependence with a withdrawal that can include seizures. Not medical advice.
Interactionsdocumented pairs only, not exhaustive
Phenobarbital is one of the strongest enzyme inducers in clinical use, and that is most of its interaction profile. It induces CYP3A4, CYP2C9, CYP1A2 and several glucuronidating enzymes, so concentrations of anything cleared by those routes fall, often by half or more. Warfarin loses effect, hormonal contraceptives fail, tacrolimus and cyclosporine troughs collapse with rejection following, direct oral anticoagulants and many antivirals drop below useful levels, and co-administered anticonvulsants such as lamotrigine are cleared faster. Induction builds over one to three weeks and takes as long to fade, so both starting and stopping phenobarbital is a hazard window.
Running the other way, valproate inhibits phenobarbital's own metabolism and can lift its concentration into the heavily sedating range. Opioids, benzodiazepines and alcohol add to respiratory depression, and barbiturates leave a narrower gap between sedation and apnea than benzodiazepines do.
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Subjective profileweighing the evidence above
Still earns its keep as a cheap, steady anticonvulsant and as a tool for managing depressant withdrawal. It is also a barbiturate: no ceiling on respiratory depression, no naloxone-style antidote, heavy enzyme induction that disturbs other medicines, and dependence with a withdrawal that can include seizures. Supervision only.
Resources
This entry is here for reference.
Research
- 1.Current position of phenobarbital in epilepsy and its future
- 2.Clinically relevant drug interactions with antiepileptic drugs
2 listed here; entry last updated July 2026
Reviews
My notesprivate to this device
FAQ
What is phenobarbital mainly used for today?
Chiefly as an anticonvulsant for seizures, and in some settings for managing withdrawal from alcohol or other sedatives.
Why does it interact with so many drugs?
It strongly induces liver enzymes, speeding the breakdown of many other medications and lowering their blood levels.
Is it safer than shorter barbiturates?
Its long action suits maintenance dosing, but it still has a narrow overdose margin and dangerous synergy with other depressants.
Adverse effects
- Sedation and cognitive dulling
- Respiratory depression in overdose
- Many drug interactions from enzyme induction
- Dependence and withdrawal