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Phenobarbital Side Effects

Medically reviewed by Last updated on Aug 25, 2023.

Applies to phenobarbital: oral elixir, oral tablets, parenteral injection.

Side effects include:

Residual sedation, drowsiness, lethargy, vertigo, nausea, vomiting, headache.

For Healthcare Professionals

Applies to phenobarbital: compounding powder, injectable solution, intravenous powder for injection, oral capsule, oral elixir, oral tablet.


The most commonly reported side effect was somnolence.[Ref]

Nervous system

Drowsiness/sedation tended to decrease with continued use.

Localized or diffuse neuralgic pain occurred, especially in psychoneurotic patients with insomnia. This pain was most frequently located in the neck, shoulders, and upper limbs and appeared in paroxysms that were most intense early in the morning. In some patients, the pain continued for days after discontinuation of this drug.

Hyperactivity occurred in pediatric and geriatric patients.[Ref]

Common (1% to 10%): Somnolence

Postmarketing reports: Sedation, residual sedation/"hangover" effect, drowsiness, lethargy, vertigo, localized/diffuse neuralgic pain, headache, hyperactivity, hypotension, hyperkinesia, ataxia, central nervous system (CNS) depression, dizziness, impairment of fine motor skills, Grand mal convulsion, prolonged coma, depressed/absent reflexes, nystagmus[Ref]


Some patients have experienced paradoxical excitement, restlessness, or delirium in the presence of pain.

Paradoxical reactions, hallucinations, restlessness, hyperexcitability, and confusion have occurred in geriatric and pediatric patients. Pediatric patients were more likely to experience aggression or irritability.[Ref]

Postmarketing reports: Accentuated emotional disturbances/phobias, aggression, excitement/hyperexcitability, restlessness, delirium, irritability, agitation, confusion, nightmares, nervousness, psychiatric disturbance, hallucinations, insomnia/sleep disturbance, anxiety, abnormality in thinking, paradoxical reaction (unusual excitement), mental depression/depression, cognitive impairment, behavioral disturbances in children, abnormal behavior, mood altered, subtle mood changes, menstrually related mood disorder, suicidal ideation, memory/concentration/judgment impairment, withdrawal syndrome, tolerance, dependence, psychic/physical dependence, mental confusion/confusion, disorientation[Ref]


Postmarketing reports: Antiepileptic hypersensitivity syndrome (e.g., fever, rash, lymphadenopathy, lymphocytosis, eosinophilia, hematological abnormalities, hepatic and other organ involvement [renal, pulmonary])[Ref]

Antiepileptic hypersensitivity syndrome typically occurred 1 to 8 weeks after first exposure or within 1 day of rechallenge. This syndrome may have cross reactivity with other antiepileptic agents.[Ref]


Postmarketing reports: Erythematous dermatitis, exfoliative dermatitis (e.g., Stevens-Johnson syndrome, toxic epidermal necrolysis), skin eruptions, maculopapular/morbilliform/scarlatiniform rashes, dermatitis, erythema multiforme, drug eruption, evidence of connective tissue changes, skin blisters/bullae, photosensitivity, erythroderma, urticaria[Ref]

Skin eruptions may be associated with fever, delirium, and marked changes in the liver and other organs.[Ref]


Postmarketing reports: Localized/diffuse myalgic or arthritic pain, osteomalacia, rickets, decreased bone mineral density, increased risk of fracture, Dupuytren's contracture, bone metabolism disorder, frozen shoulder, Ledderhose's syndrome, general joint pain, osteopenia, osteoporosis, fractures[Ref]

Localized or diffuse myalgic or arthritic pain occurred, especially in psychoneurotic patients with insomnia. This pain was most frequently located in the neck, shoulders, and upper limbs and appeared in paroxysms that were most intense early in the morning. In some patients, pain continued for days after discontinuation of therapy.

Decreased bone mineral density, Dupuytren's contracture, osteopenia, osteoporosis, and fractures occurred in patients receiving long-term therapy; however, the exact mechanism of action in bones was not identified.[Ref]


Postmarketing reports: Inebriation (drunk-like effect), fever, neonatal sedation, neonatal drug dependence/withdrawal, neonatal symptoms resembling vitamin K deficiency, neonatal bleeding due to vitamin K deficiency, congenital anomaly, cleft lip and palate, lowered body temperature[Ref]


Postmarketing reports: Megaloblastic anemia, macrocytic anemia, aplastic anemia, agranulocytosis, thrombocytopenia, hypoprothrombinemia, methemoglobinemia, purpura, lymphocytosis[Ref]

Megaloblastic anemia occurred after chronic use of this drug, and may be due to folate deficiency.

Methemoglobinemia has occurred in infants nursed by mothers receiving this drug.[Ref]


Postmarketing reports: Circulatory collapse, peripheral vascular collapse, weak heartbeat, circulatory failure, bradycardia, hypotension, profound shock, vasodilation[Ref]


Postmarketing reports: Respiratory depression, severe/significant respiratory depression, apnea, hypoventilation, bronchospasm, laryngospasm[Ref]

Bronchospasm and laryngospasm were reported, especially in patients given the IV formulation.[Ref]


Postmarketing reports: Abnormal hepatic function, hepatitis, liver damage, cholestasis, toxic hepatitis, jaundice[Ref]


Postmarketing reports: Hypocalcemia, folate deficiency, abnormal vitamin D metabolism, increased vitamin D requirements (possibly resulting from abnormal vitamin D metabolism), vitamin K deficiency, hypophosphatemia[Ref]


Postmarketing reports: Nausea, vomiting, constipation, diarrhea[Ref]


Postmarketing reports: Localized swelling (e.g. eyelids, cheeks, lips), hypersensitivity reactions (e.g., angioedema, skin rashes, exfoliative dermatitis), angioedema[Ref]

Hypersensitivity reactions generally consisted of acquired hypersensitivity. Patients with a higher risk of developing reactions were more likely to have asthma, urticaria, and angioedema.[Ref]


Postmarketing reports: Injection site reactions, local necrosis after extravasation (IV/subcutaneous injection)[Ref]


Postmarketing reports: Renal failure[Ref]


Postmarketing reports: Fibromas[Ref]


Postmarketing reports: Reduced serum concentrations of thyroid hormones[Ref]


Postmarketing reports: Peyronie's disease[Ref]


1. Product Information. Phenobarbital (phenobarbital). Lilly, Eli and Company. 2001.

2. Cerner Multum, Inc. UK Summary of Product Characteristics.

3. Cerner Multum, Inc. Australian Product Information.

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

Some side effects may not be reported. You may report them to the FDA.