Phenobarbital Side Effects

Please note - some side effects for Phenobarbital may not be reported. Always consult your doctor or healthcare specialist for medical advice. You may also report side effects to the FDA at http://www.fda.gov/medwatch/ or 1-800-FDA-1088 (1-800-332-1088).

Side Effects of Phenobarbital - for the Consumer

Phenobarbital

All medicines may cause side effects, but many people have no, or minor side effects. Check with your doctor if any of these most COMMON side effects persist or become bothersome when using Phenobarbital:

Agitated mood; clumsiness; confusion; dizziness; excessive daytime drowsiness; headache; injection site reactions; lightheadedness; low blood pressure; nausea; slow heartbeat; slowed breathing; vomiting.

Severe allergic reactions (rash; hives; itching; difficulty breathing; tightness in the chest; swelling of the mouth, face, lips, or tongue); difficulty breathing; liver damage.

Phenobarbital Elixir

All medicines may cause side effects, but many people have no, or minor side effects. Check with your doctor if any of these most COMMON side effects persist or become bothersome when using Phenobarbital Elixir:

Clumsiness; dizziness; drowsiness; excessive daytime drowsiness ("hangover effect"); feeling of a whirling motion; headache; lightheadedness; nausea; tired feeling; vomiting; weak bones.

Severe allergic reactions (rash; hives; itching; difficulty breathing; tightness in the chest; swelling of the mouth, face, lips, or tongue); confusion; difficulty sleeping; fainting; intense pain; very slow breathing.

Phenobarbital Tablets

All medicines may cause side effects, but many people have no, or minor side effects. Check with your doctor if any of these most COMMON side effects persist or become bothersome when using Phenobarbital Tablets:

Clumsiness; dizziness; drowsiness; excessive daytime drowsiness ("hangover effect"); feeling of a whirling motion; headache; lightheadedness; nausea; tired feeling; vomiting; weak bones.

Severe allergic reactions (rash; hives; itching; difficulty breathing; tightness in the chest; swelling of the mouth, face, lips, or tongue); confusion; difficulty sleeping; fainting; intense pain; very slow breathing.

Side Effects by Body System

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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