Phenobarbital Side Effects

Some side effects of phenobarbital may not be reported. Always consult your doctor or healthcare specialist for medical advice. You may also report side effects to the FDA.

For the Consumer

Applies to phenobarbital: oral capsule, oral elixir, oral tablet

Phenobarbital may cause a severe allergic reaction. Stop taking phenobarbital and Get emergency medical help if you have any of these signs of an allergic reaction while taking phenobarbital: hives; difficulty breathing; swelling of your face, lips, tongue, or throat.

Call your doctor at once if you have any of these serious side effects:

• restless muscle movements in your eyes, tongue, jaw, or neck;

• slow heartbeat, shallow breathing;

• feeling light-headed, fainting;

• a fever or a sore throat;

• sores in your mouth;

• easy bruising or bleeding; or

• broken blood vessels under your skin.

Less serious side effects of phenobarbital may include:

• drowsiness or dizziness;

• problems with memory or concentration;

• excitement, irritability, aggression, or confusion (especially in children or older adults);

• loss of balance or coordination;

• nausea, constipation;

• headache; or

• "hangover" effect (drowsiness the day after a dose).

This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects.

For Healthcare Professionals

Applies to phenobarbital: compounding powder, injectable solution, oral capsule, oral elixir, oral tablet

Nervous system

Nervous system side effects including drowsiness, sedation, lethargy, somnolence, memory impairment, decreased perceptuomotor performance, and impairment of tasks requiring sustained performance have been reported frequently. Paradoxical stimulation and irritability, behavioral problems, impaired cognition, tics, dyskinesias, periodic alternating nystagmus, oculogyric crises, dystonic reactions, and ataxia have also been reported.

Respiratory

Respiratory side effects including life threatening respiratory depression have been reported, particularly during acute intoxication.

Other

Other side effects from phenobarbital include physical and psychological dependence. Withdrawal symptoms may occur if phenobarbital therapy is discontinued suddenly. An increased frequency of seizures may also occur if phenobarbital therapy is suddenly discontinued. Dupuytren's contractures have also been reported.

Withdrawal symptoms may include sweating, insomnia, hallucinations, and hypertension.

Musculoskeletal

Musculoskeletal side effects including reflex sympathetic dystrophy of the upper extremities, the "shoulder-hand syndrome", and other musculoskeletal and joint pains have been reported in as many as 5% of patients treated with phenobarbital. Myopathy in association with anticonvulsant osteomalacia has also been reported.

Hypersensitivity

Hypersensitivity side effects including generalized rashes, fever, and hepatic derangements have been reported frequently. Nephritis has also been reported as a consequence of hypersensitivity to phenobarbital. A case of a fatal anticonvulsant hypersensitivity syndrome has been reported in an infant.

Hepatic

Hepatic side effects including elevated liver function tests, hepatitis, cholestasis, and fulminant hepatic failure have been reported. Mild enzyme elevation is common and often represents enzyme induction and not hepatic pathology.

The more serious hepatic effects have often been reported in association with hypersensitivity reactions.

Dermatologic

Dermatologic side effects including toxic epidermal necrolysis, pemphigus vulgaris, photosensitivity, acneiform rash, and purpura have been reported.

Gastrointestinal

Gastrointestinal side effects including a case of esophagitis have been reported.

Hematologic

Hematologic side effects including macrocytosis and, more rarely, megaloblastic anemia have been reported.

Endocrine

Endocrine side effects including alterations in vitamin D metabolism have been reported in patients taking phenobarbital and other anticonvulsants. Hypocalcemia and osteomalacia have also been reported.

Metabolic

Metabolic side effects including an increase in the hepatic metabolism of thyroxine (T4) and triiodothyronine (T3) have been reported.

Oncologic

Oncologic side effects including an increased frequency of malignancy (particularly brain tumors, lung cancer, and liver cancer) have been reported. However, other investigators have not found an association with malignancy.

Genitourinary

Genitourinary side effects including Peyronie's disease have been reported.

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