From the Guidelines
Phenobarbital can cause several severe side effects, including profound respiratory depression and hypotension, which can be life-threatening, as well as less severe effects such as drowsiness, dizziness, and impaired coordination, emphasizing the need for cautious use and monitoring 1.
Common Side Effects
- Drowsiness
- Dizziness
- Fatigue
- Impaired coordination
- Cognitive effects such as confusion, memory problems, and difficulty concentrating
- Gastrointestinal issues like nausea, vomiting, and constipation
- Mood changes including depression, irritability, or paradoxical excitement
Serious Side Effects
- Allergic reactions (rash, itching, swelling)
- Unusual bleeding or bruising
- Yellowing of the skin or eyes (jaundice)
- Persistent nausea/vomiting
- Dark urine
- Severe dizziness
- Profound respiratory depression and hypotension, which can be life-threatening 1
Long-term Use Considerations
Long-term use of phenobarbital may lead to physical dependence, making it crucial to never stop taking the medication suddenly, as withdrawal can be dangerous 1.
Mechanism and Interactions
Phenobarbital works by enhancing the inhibitory effects of GABA in the brain, which slows down central nervous system activity, and it can interact with many medications by affecting liver enzymes, potentially reducing the effectiveness of birth control pills, anticoagulants, and other drugs 1.
From the FDA Drug Label
ADVERSE REACTIONS The following adverse reactions and their incidence were compiled from surveillance of thousands of hospitalized patients. Because such patients may be less aware of certain of the milder adverse effects of barbiturates, the incidence of these reactions may be somewhat higher in fully ambulatory patients Nervous System Somnolence, agitation, confusion, hyperkinesia, ataxia, CNS depression, nightmares, nervousness, psychiatric disturbance, hallucinations, insomnia, anxiety, dizziness, thinking abnormality Respiratory System Hypoventilation, apnea Cardiovascular System Bradycardia, hypotension, syncope Digestive System Nausea, vomiting, constipation Dermatologic Reactions Exfoliative dermatitis, Stevens-Johnson syndrome, toxic epidermic necrolysis. Other Reported Reactions Headache; injection site reactions; hypersensitivity reactions, including but not limited to angioedema and skin rashes; fever; liver damage and megaloblastic anemia following chronic phenobarbital use.
The side effects of phenobarbital include:
- Nervous System: somnolence, agitation, confusion, hyperkinesia, ataxia, CNS depression, nightmares, nervousness, psychiatric disturbance, hallucinations, insomnia, anxiety, dizziness, thinking abnormality
- Respiratory System: hypoventilation, apnea
- Cardiovascular System: bradycardia, hypotension, syncope
- Digestive System: nausea, vomiting, constipation
- Dermatologic Reactions: exfoliative dermatitis, Stevens-Johnson syndrome, toxic epidermic necrolysis
- Other Reported Reactions: headache, injection site reactions, hypersensitivity reactions, fever, liver damage, and megaloblastic anemia following chronic use 2
From the Research
Side Effects of Phenobarbital
The side effects of phenobarbital have been studied in various research papers. Some of the common side effects include:
- Behavioral disorders, such as excitement and depression 3
- Sedation 4
- Neurologic disorders 3
- Digestive tract disorders 3
- Gingival hyperplasia and hirsutism 3
Comparison with Other Antiepileptic Drugs
Phenobarbital has been compared with other antiepileptic drugs, such as phenytoin, carbamazepine, and valproate, in terms of its side effects. The results show that:
- Phenobarbital is more likely to cause behavioral disorders, such as hyperactivity and behavioral problems, than other antiepileptic drugs 4
- Phenytoin is more likely to cause sedation, cerebellar syndrome, and phenytoin encephalopathy than phenobarbital 4
- Valproate is more likely to cause digestive tract disorders and liver disease than phenobarbital 3
- Carbamazepine is less likely to cause side effects than phenobarbital and phenytoin 3, 5
Cognitive Effects
The cognitive effects of phenobarbital have also been studied. The results show that:
- Phenobarbital can impair cognitive function, particularly in areas such as choice reaction time, finger tapping, and paragraph memory 6
- Phenobarbital is more likely to cause cognitive impairment than phenytoin and valproate 6
- The cognitive effects of phenobarbital are dose-related, with higher doses causing more severe impairment 4