Phenobarbital Side Effects

It is possible that some side effects of phenobarbital may not have been reported. These can be reported to the FDA here. Always consult a healthcare professional for medical advice.

For the Consumer

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, 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

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

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.

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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