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The following adverse reactions and their incidence were compiled from surveillance of thousands of hospitalized patients.
- Nervous system: Somnolence.
- Nervous system: 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.
- Other reported reactions: Headache, injection site reactions, hypersensitivity reactions (angioedema skin rashes, exfoliative dermatitis), fever, liver damage, megaloblastic anemia following chronic phenobarbital use.
DRUG ABUSE AND DEPENDENCE
Symptoms of acute intoxication with phenobarbital include unsteady gait, slurred speech, and sustained nystagmus. Mental signs of chronic intoxication include confusion, poor judgment, irritability, insomnia, and somatic complaints.
Symptoms of phenobarbital dependence are similar to those of chronic alcoholism. If an individual appears to be intoxicated with alcohol to a degree that is radically disproportionate to the amount of alcohol in his or her blood, the use of barbiturates should be suspected. The lethal dose of a barbiturate is far less if alcohol is also ingested. The symptoms of phenobarbital withdrawal can be severe and may cause death. Minor withdrawal symptoms may appear 8 to 12 hours after the last dose of phenobarbital. These symptoms usually appear in the following order: anxiety, muscle twitching, tremor of hands and fingers, progressive weakness, dizziness, distortion in visual perception, nausea, vomiting, insomnia, and orthostatic hypotension. Major withdrawal symptoms (convulsions and delirium) may occur within 16 hours and last up to 5 days after abrupt cessation of this drug. Intensity of withdrawal symptoms gradually declines over a period of approximately 15 days. Individuals susceptible to phenobarbital abuse and dependence include alcoholics and opiate abusers, as well as other sedative- hypnotic and amphetamine abusers.
Drug dependence on phenobarbital arises from repeated administration of the barbiturate or an agent with barbiturate- like effect on a continuous basis, generally in amounts exceeding therapeutic dose levels. The characteristics of drug dependence on phenobarbital include: (a) a strong desire or need to continue taking the drug, (b) a tendency to increase the dose, (c) a psychic dependence on the effects of the drug related to subjective and individual appreciation of those effects, and (d) a physical dependence on the effects of the drug requiring its presence for maintenance of homeostasis and resulting in a definite, characteristic, and self-limited abstinence syndrome when the drug is withdrawn.
Treatment of phenobarbital dependence consists of cautious and gradual withdrawal of the drug. One method involves substituting a 30 mg dose of phenobarbital for each 100 to 200 mg dose that the patient has been taking. The total daily amount of phenobarbital is then administered in 3 to 4 divided doses, not to exceed 600 mg daily. Should signs of withdrawal occur on the first day of treatment, a loading dose of 100 to 200 mg of phenobarbital may be administered IM in addition to the oral dose. After stabilization on phenobarbital, the total daily dose is decreased by 30 mg a day as long as withdrawal is proceeding smoothly. A modification of this regimen involves initiating treatment at the patient's regular dosage level and decreasing the daily dosage by 10 percent if tolerated by the patient.
Infants physically dependent on phenobarbital may be given a lower dose of phenobarbital at 3 to 10 mg/kg/day. After withdrawal symptoms (hyperactivity, disturbed sleep, tremors, hyperreflexia) are relieved, the dosage of phenobarbital should be gradually decreased and completely withdrawn over a 2-week period.
Read the Phenobarbital (phenobarbital) Side Effects Center for a complete guide to possible side effects
Most reports of clinically significant drug interactions occurring with the barbiturates have involved phenobarbital.
1. Anticoagulants: Phenobarbital lowers the plasma levels of dicumarol (name previously used: bishydorxycoumarin) and causes a decrease in anticoagulant activity as measured by the prothrombin time. Phenobarbital can induce hepatic microsomal enzymes resulting in increased metabolism and decreased anticoagulant response of oral anticoagulants (e.g., warfarin, acenocournarol, dicumarol, and phenprocoumon). Patients stabilized on anticoagulant therapy may require dosage adjustments if phenobarbital is added to or withdrawn from their dosage regimen.
2. Corticosteroids: Phenobarbital appears to enhance the metabolism of exogenous corticosteroids probably through the induction of hepatic microsomal enzymes. Patients stabilized on corticosteroid therapy may require dosage adjustments if phenobarbital is added to or withdrawn from their dosage regimen.
3. Griseofulvin: Phenobarbital appears to interfere with the absorption of orally administered griseofulvin, thus decreasing its blood level. The effect of the resultant decreased blood levels of griseofulvin on therapeutic response has not been established. However, it would be preferable to avoid concomitant administration of these drugs.
4. Doxycycline: Phenobarbital has been shown to shorten the half- life of doxycycline for as long as 2 weeks after barbiturate therapy is discontinued. This mechanism is probably through the induction of hepatic microsomal enzymes that metabolize the antibiotic. If phenobarbital and doxycycline are administered concurrently, the clinical response to doxycycline should be monitored closely.
5. Phenytoin, sodium valproate, valproic acid: The effect of phenobarbital on the metabolism of phenytoin appears to be variable. Some investigators report an accelerating effect, while others report no effect. Because the effect of phenobarbital on the metabolism of phenytoin is not predictable, phenytoin and phenobarbital blood levels should be monitored more frequently if these drugs are given concurrently. Sodium valproate and valproic acid appear to decrease phenobarbital metabolism; therefore, phenobarbital blood levels should be monitored and appropriate dosage adjustments made as indicated.
6. Central nervous system depressants: The concomitant use of other central nervous system depressants including other sedatives or hypnotics, antihistamines, tranquilizers, or alcohol, may produce additive depressant effects.
7. Monoamine oxidase inhibitors (MAOIs): MAOIs prolong the effects of phenobarbital probably because metabolism of the phenobarbital is inhibited.
8. Estradiol, estrone, progesterone and other steroidal hormones: Pretreatment with or concurrent administration of phenobarbital may decrease the effect of estradiol by increasing its metabolism. There have been reports of patients treated with antiepileptic drugs (e.g., phenobarbital) who became pregnant while taking oral contraceptives. An alternate contraceptive method might be suggested to women taking phenobarbital.This monograph has been modified to include the generic and brand name in many instances.
Last reviewed on RxList: 1/17/2017
Additional Phenobarbital Information
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