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.

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.

Seek medical attention right away if any of these SEVERE side effects occur when using Phenobarbital:

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

This is not a complete list of all side effects that may occur. If you have questions about side effects, contact your health care provider. Call your doctor for medical advice about side effects. To report side effects to the appropriate agency, please read the Guide to Reporting Problems to FDA.

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.

Seek medical attention right away if any of these SEVERE side effects occur when using Phenobarbital Elixir:

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.

This is not a complete list of all side effects that may occur. If you have questions about side effects, contact your health care provider. Call your doctor for medical advice about side effects. To report side effects to the appropriate agency, please read the Guide to Reporting Problems to FDA.

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.

Seek medical attention right away if any of these SEVERE side effects occur when using Phenobarbital Tablets:

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.

This is not a complete list of all side effects that may occur. If you have questions about side effects, contact your health care provider. Call your doctor for medical advice about side effects. To report side effects to the appropriate agency, please read the Guide to Reporting Problems to FDA.

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Phenobarbital Side Effects - for the Professional

Phenobarbital

The following adverse reactions have been reported:

CNS Depression – Residual sedation or “hangover”, drowsiness, lethargy, and vertigo. Emotional disturbances and phobias may be accentuated. In some persons, barbiturates such as Phenobarbital repeatedly produce excitement rather than depression, and the patient may appear to be inebriated. Irritability and hyperactivity can occur in children. Like other nonanalgesic hypnotic drugs, barbiturates such as Phenobarbital, when given in the presence of pain, may cause restlessness, excitement, and even delirium. Rarely, the use of barbiturates results in localized or diffuse myalgic, neuralgic, or arthritic pain, especially in psychoneurotic patients with insomnia. The pain may appear in paroxysms, is most intense in the early morning hours, and is most frequently located in the region of the neck, shoulder girdle, and upper limbs. Symptoms may last for days after the drug is discontinued.

Respiratory/Circulatory – Respiratory depression, apnea, circulatory collapse.

Allergic – Acquired hypersensitivity to barbiturates consists chiefly in allergic reactions that occur especially in persons who tend to have asthma, urticaria, angioedema, and similar conditions. Hypersensitivity reactions in this category include localized swelling, particularly of the eyelids, cheeks, or lips, and erythematous dermatitis. Rarely, exfoliative dermatitis (e.g., Stevens-Johnson syndrome and toxic epidermal necrolysis) may be caused by Phenobarbital and can prove fatal. The skin eruption may be associated with fever, delirium, and marked degenerative changes in the liver and other parenchymatous organs. In a few cases, megaloblastic anemia has been associated with the chronic use of Phenobarbital.

Other – Nausea and vomiting; headache, osteomalacia.

The following adverse reactions and their incidence were compiled from surveillance of thousands of hospitalized patients who received barbiturates. Because such patients may be less aware of the milder adverse effects of barbiturates, the incidence of these reactions may be somewhat higher in fully ambulatory patients.

More than 1 in 100 Patients: The most common adverse reaction, estimated to occur at a rate of 1 to 3 patients per 100, is:

Nervous System: Somnolence.

Less than 1 in 100 Patients: Adverse reactions estimated to occur at a rate of less than 1 in 100 patients are listed below, grouped by organ system and by decreasing order of occurrence:

Nervous System: Agitation, confusion, hyperkinesia, ataxia, CNS depression, nightmares, nervousness, psychiatric disturbance, hallucinations, insomnia, anxiety, dizziness, abnormality in thinking.

Respiratory System: Hypoventilation, apnea

Cardiovascular System: Bradycardia, hypotension, syncope

Digestive System: Nausea, vomiting, constipation

Other Reported Reactions: Headache, injection site reactions, hypersensitivity reactions (angioedema, skin rashes, exfoliative dermatitis), fever, liver damage, megaloblastic anemia following chronic Phenobarbital use

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Side Effects by Body System - for Healthcare Professionals

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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