Short answer: Phenobarbital is a long-acting barbiturate used mainly for seizures and, in hospitals, for alcohol withdrawal. It is one of the oldest medicines still in use. It is a Schedule IV controlled substance with a narrow margin between a helpful and a dangerous dose.
Key takeaways
- What it is: Phenobarbital is a barbiturate and a Schedule IV controlled substance in the US.
- Main uses: Seizures; alcohol withdrawal.
- Timing: effects start in about 1 hour and last 10 to 12 hours or more.
- Most dangerous mix: Opioids, which can cause slowed breathing, death.
- Naloxone: it does not reverse this drug, but give it whenever opioids could be involved.
- Dependence risk: High.
- Legal route: only with a prescription from a DEA-registered clinician, filled by a licensed pharmacy.
What is phenobarbital?
Phenobarbital is used to prevent and treat seizures and, in hospitals, increasingly to manage severe alcohol withdrawal. Most oral phenobarbital in the US is sold under older, unapproved-drug status; Sezaby is an FDA-approved injection for seizures in newborns.
It is inexpensive and recommended by the World Health Organization as an essential antiseizure medicine, but newer medicines are usually preferred because of sedation, interactions, and overdose risk.
Quick facts
| Fact | Phenobarbital |
|---|---|
| Generic name | Phenobarbital |
| Brand names | Sezaby (injection for newborn seizures); most products are unbranded |
| Drug class | Barbiturate |
| US legal status | Schedule IV, prescription only |
| Approved uses | Seizures; sedation (older labeling); neonatal seizures (Sezaby) |
| Forms | Tablets, elixir, injection |
| Onset | About 1 hour by mouth |
| Half-life | About 79 hours (range 53 to 118) |
| Broken down by | Liver (CYP2C9, CYP2C19); strongly induces other liver enzymes |
How long does phenobarbital last?
- Starts working: About 1 hour
- Effects last: 10 to 12 hours or more
- Half-life: About 79 hours (range 53 to 118)
Its half-life averages about 79 hours, so it takes 2 to 3 weeks to clear fully.
Timing varies with the dose, the form, how it is taken, liver and kidney function, age, and other drugs taken at the same time.
What are the biggest risks of phenobarbital?
Common side effects include drowsiness, slowed thinking, depression, and, in children and older adults, paradoxical hyperactivity or confusion.
Serious risks include fatal overdose, especially with alcohol or opioids; serious skin reactions; bone thinning with long-term use; and birth defects.
Warning: Unlike benzodiazepines, barbiturates have no ceiling on their sedative effect. Overdose can stop breathing even without other drugs, and there is no antidote.
What should you never mix with phenobarbital?
| Combined with | What happens | Label guidance |
|---|---|---|
| Opioids | Slowed breathing, death | Avoid unless no alternative |
| Alcohol | Additive sedation; overdose | Avoid |
| Hormonal birth control | Lower contraceptive levels; pregnancy | Use another method |
| Warfarin, many antiseizure drugs, steroids, some HIV drugs | Phenobarbital speeds their breakdown | Monitor levels; adjust doses |
| Valproate | Raises phenobarbital levels | Monitor |
Check any combination with the DoseUses combination checker or ask a pharmacist.
What are the signs of a phenobarbital overdose, and what should you do?
Phenobarbital overdose causes deep sleepiness, confusion, slurred speech, loss of reflexes, low body temperature, slowed breathing, low blood pressure, and coma. It is more dangerous than benzodiazepine overdose because there is no reversal drug.
If someone may have overdosed
- Call 911 if the person is unresponsive, hard to wake, or breathing slowly.
- For advice when the person is awake and stable, call Poison Help at 1-800-222-1222 (US, free, 24 hours).
- If opioids could be involved, including pills not from a pharmacy, give naloxone if you have it. Naloxone does not reverse phenobarbital, but it will not harm the person.
- Stay with the person and place them on their side if they are drowsy or vomiting.
What is phenobarbital withdrawal like, and how long does it last?
Regular use causes physical dependence. Barbiturate withdrawal is among the most dangerous drug withdrawals.
- Days 2 to 4Symptoms beginAnxiety, tremor, weakness, insomnia, nausea.
- Days 3 to 8Severe withdrawalSeizures and delirium can occur and can be fatal.
- WeeksRecoveryGradual return of sleep and mood.
See the withdrawal timelines and the full phenobarbital withdrawal guide.
Is phenobarbital legal?
In the United States, phenobarbital is a Schedule IV controlled substance. The only legal way to get it is with a prescription from a clinician registered with the Drug Enforcement Administration (DEA), filled by a licensed pharmacy. Depending on state law, that can be a physician, nurse practitioner, or physician assistant.
- Refills: a Schedule IV prescription can be refilled up to five times within six months of the date it was written. After that, a new prescription is needed.
- Telehealth: federal flexibilities let DEA-registered clinicians prescribe Schedule II to V medicines after a telemedicine visit, without a prior in-person exam, through December 31, 2026. A permanent DEA rule was under final review in late 2026. State rules can be stricter.
- Monitoring: prescribers and pharmacists in most states check a prescription drug monitoring program, a database of controlled-substance prescriptions, before prescribing or dispensing.
Outside the United States
| Country | Status |
|---|---|
| United Kingdom | Class B; prescription only |
| Canada | Schedule IV; prescription only |
| Australia | Schedule 4 prescription medicine |
| International | Schedule IV of the 1971 Convention on Psychotropic Substances |
Frequently asked questions about phenobarbital
Is phenobarbital still used?
Yes, for seizures, especially in newborns and where cost matters, and increasingly in hospitals for alcohol withdrawal.
Is phenobarbital a benzodiazepine?
No. It is a barbiturate. Barbiturates are more dangerous in overdose because they have no ceiling effect and no antidote.
Does phenobarbital affect birth control?
Yes. It speeds up the breakdown of hormonal contraceptives and can make them fail.
How long does phenobarbital stay in your system?
Its half-life averages about 79 hours, so it takes 2 to 3 weeks to clear fully.
Sources
- Sezaby (phenobarbital) prescribing information, DailyMed, US National Library of Medicine.
- Controlled Substance Schedules, DEA Diversion Control Division.
- 21 CFR 1306.22, Refilling of prescriptions (Schedules III and IV), Electronic Code of Federal Regulations.
- Prescribing controlled substances via telehealth, US Department of Health and Human Services.
- BeSafeRx: Your Source for Online Pharmacy Information, US Food and Drug Administration.
- Safe Pharmacy, National Association of Boards of Pharmacy.
- One Pill Can Kill, US Drug Enforcement Administration.
- International drug control conventions, United Nations Office on Drugs and Crime.
This guide summarizes the sourced DoseUses Phenobarbital profile, written from FDA labels, DEA rules, and federal health agencies. It is educational and is not medical advice. Last updated October 5, 2026.
