Phenobarbital

Patient Information

Practitioners should give the following information and instructions to patients receiving barbiturates:

1. The use of phenobarbital carries with it an associated risk of psychological and/or physical dependence. The patient should be warned against increasing the dose of the drug without consulting a physician.

2. Phenobarbital may impair mental and/or physical abilities required for the performance of potentially hazardous tasks (e.g., driving, operating machinery, etc.).

3. Alcohol should not be consumed while taking phenobarbital. Concurrent use of phenobarbital with other CNS depressants (e.g., alcohol, narcotics, tranquilizers, and antihistamines) may result in additional CNS depressant

Description

Phenobarbital is a barbiturate, nonselective central nervous system depressant which is primarily used as a sedative hypnotic and also as an anticonvulsant in subhypnotic doses.

Phenobarbital Tablets and Elixir are administered orally and are contained in DEA Schedule IV. Barbiturates are substituted pyrimidine derivatives in which the basic structure common to these drugs is barbituric acid, a substance which has no central nervous system (CNS) activity. CNS activity is obtained by substituting alkyl, alkenyl, or aryl groups on the pyrimidine ring.

Chemically Designated: 5-Ethyl -5-phenylbarbituric acid

Molecular Formula: C₁₂H₁₂N₂O₃

Molecular Weight: 232.24

Inactive Ingredients

Oral Tablets – corn starch, lactose (monohydrate), magnesium stearate and sodium starch glycolate

Oral Elixir – ethyl alcohol, glycerin, oil of orange, sucrose, water, FD& C Red #40 and FD& C Blue #1

Oral

a. Sedatives.

b. Hypnotics, for the short-term treatment of insomnia, since they appear to lose their effectiveness for sleep induction and sleep maintenance after 2 weeks. (See CLINICAL PHARMACOLOGY .)

c. Preanesthetics.

d. Long-term anticonvulsants for the treatment of generalized tonic-clonic and cortical local seizures. And, in the emergency control of certain acute convulsive episodes, e.g., those associated with status epilepticus, cholera, eclampsia, meningitis, tetanus, and toxic reactions to strychnine or local anesthetics.

Dosage And Admintisration

Suggested doses of phenobarbital for specific indications are as follows:

a. Pediatric Oral Dosage (as recommended by the American Academy of Pediatrics):

Preoperative: 1 to 3 mg/kg.

b. Adult Oral Dosage:

1. Daytime sedative: 30 to 120 mg daily in 2 to 3 divided doses.2. Bedtime hypnotic: 100 to 320 mg.3. Anticonvulsant: 50 to 100 mg 2 to 3 times daily.Dosages of phenobarbital must be individualized with full knowledge of their particular characteristics and recommended rate of administration. Factors of consideration are the patient’s age, weight, and condition. Parenteral routes should be used only when oral administration is impossible or impractical.

Anticonvulsant use: A therapeutic anticonvulsant level of phenobarbital in serum is 10 to 25 µg/mL. To achieve the blood levels considered therapeutic in children, higher per-kilogram dosages are generally necessary for phenobarbital and most other anticonvulsants. In children and infants, phenobarbital at loading dose of 15 to 20 mg/kg produces blood levels of about 20 µg/mL shortly after administration.

In status epilepticus, it is imperative to achieve therapeutic blood levels of phenobarbital as rapidly as possible. Because a barbiturate-induced depression may occur along with a postictal depression once the seizures are controlled, it is important, therefore, to use the minimal amount required, and to wait for the anticonvulsant effect to develop before administering a second dose.

Phenobarbital has been used in the treatment and prophylaxis of febrile seizures. However, it has not been established that prevention of febrile seizures influences the subsequent development of epilepsy.

Special patient population: Dosage should be reduced in the elderly or debilitated because these patients may be more sensitive to phenobarbital. Dosage should be reduced for patients with impaired renal function or hepatic disease.

How Supplied

Oral Tablets

15 mg – Each white round tablet imprinted Þ 026 contains 15 mg of Phenobarbital. Tablets are supplied in bottles of 1000 (NDC 0228-2026-96).

30 mg – Each white, round, scored tablet imprinted Þ 028 contains 30 mg of Phenobarbital. Tablets are supplied in bottles of 1000 (NDC 0228-2028-96).

100 mg – Each white, round, scored tablet imprinted Þ 030 contains 100 mg of Phenobarbital. Tablets are supplied in bottles of 1000 (NDC 0228-2030-96).

Dispense in well-closed containers as defined in the USP. Store at controlled room temperature 15º- 30º C (59º- 86º F).

Oral Elixir

Red, clear elixir contains 20 mg of Phenobarbital per teaspoon (5 ml). Alcohol 13% by volume. Elixir is supplied in pints (NDC 0228-2024-16).

Preserve and dispense in tight, light- resistant containers as defined in the USP. Store at controlled room temperature 15º- 30ºC (59º- 86º F).

Rx only.

The following adverse reactions and their incidence were compiled from surveillance of thousands of hospitalized 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 listed below, grouped by organ system, and by decreasing order of occurrence are:

• 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

Phenobarbital may be habit forming:Tolerance, psychological dependence, and physical dependence may occur especially following prolonged use of high doses of phenobarbital. As tolerance to phenobarbital develops, the amount needed to maintain the same level of intoxication increases; tolerance to a fatal dosage, however, does not increase more than two-fold. As this occurs, the margin between an intoxicating dosage and fatal dosage becomes smaller.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.

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.

1. Habit forming: Phenobarbital may be habit forming. Tolerance, psychological and physical dependence may occur with continued use. (See ADVERSE REACTIONS: Drug Abuse and Dependence). To minimize the possibility of overdosage or the development of dependence, the prescribing and dispensing of sedative-hypnotic barbiturates should be limited to the amount required for the interval until the next appointment. Abrupt cessation after prolonged use in the dependent person may result in withdrawal symptoms, including delirium, convulsions, and possibly death. Phenobarbital should be withdrawn gradually. (See ADVERSE REACTIONS: Drug Abuse and Dependence.)

2. Acute or chronic pain: Caution should be exercised when Phenobarbital is administered to patients with acute or chronic pain, because paradoxical excitement could be induced or important symptoms could be masked. However, the use of phenobarbital as a sedative in the postoperative surgical period and as an adjunct to cancer chemotherapy is well established.

3. Use in pregnancy: Phenobarbital can cause fetal damage when administered to a pregnant woman. Retrospective case-controlled studies have suggested a connection between the maternal consumption of phenobarbital and higher than expected incidence of fetal abnormalities. Following oral administration, Phenobarbital readily crosses the placental barrier and is distributed throughout fetal tissues with highest concentrations found in the placenta, fetal liver, and brain.

Withdrawal symptoms occur in infants born to mothers who receive phenobarbital throughout the last trimester of pregnancy. (See ADVERSE REACTIONS: Drug Abuse and Dependence.) If this drug is used during pregnancy, or if the patient becomes pregnant while taking this drug, the patient should be apprised of the potential hazard to the fetus.

4. Synergistic effects: The concomitant use of alcohol or other CNS depressants may produce additive CNS depressant effects.

General

Phenobarbital may be habit forming. Tolerance and psychological and physical dependence may occur with continuing use. (See ADVERSE REACTIONS: Drug Abuse and Dependence.) Phenobarbital should be administered with caution, if at all, to patients who are mentally depressed, have suicidal tendencies, or a history of drug abuse. Elderly or debilitated patients may react to Phenobarbital with marked excitement, depression, and confusion. In some persons, phenobarbital repeatedly produces excitement rather than depression.

In patients with hepatic damage, phenobarbital should be administered with caution and initially reduced doses. Phenobarbital should not be administered to patients showing the premonitory signs of hepatic coma.

Laboratory Tests

Prolonged therapy with phenobarbital should be accompanied by periodic laboratory evaluation of organ systems, including hematopoietic, renal, and hepatic systems (See General – above and ADVERSE REACTIONS.)

Carcinogenesis

Human-data: A retrospective study of 84 children with brain tumors matched to 73 normal controls and 78 cancer controls (malignant disease other than brain tumors) suggested an association between exposure to barbiturates prenatally and an increased incidence of brain tumors.

Pregnancy

Teratogenic effects: Pregnancy Category D. (See

WARNINGS

: Use in Pregnancy.) 

Nonteratogenic effects: Reports of infants suffering from long- term phenobarbital exposure in utero included the acute withdrawal syndrome of seizures and hyperirritability from birth to a delayed onset of up to 14 days. (See ADVERSE REACTIONS: Drug Abuse and Dependence.)

Labor and Delivery

Hypnotic doses of phenobarbital do not appear to significantly impair uterine activity during labor. Full anesthetic doses of phenobarbital decrease the force and frequency of uterine contractions. Administration of sedative-hypnotic phenobarbital to the mother during labor may result in respiratory depression in the newborn. Premature infants are particularly susceptible to the depressant effects of phenobarbital. If phenobarbital is used during labor and delivery, resuscitation equipment should be available.

Data are currently not available to evaluate the effect of phenobarbital on the later growth, development, and functional maturation of the child.

Nursing Mothers

Caution should be exercised when phenobarbital is administered to a nursing woman since small amounts of phenobarbital are excreted in the milk.

Overdose

The toxic dose of barbiturates varies considerably. In general, an oral dose of 1 gram of most barbiturates produces serious poisoning in an adult. Death commonly occurs after 2 to 10 grams of ingested barbiturate. Barbiturate intoxication may be confused with alcoholism, bromide intoxication, and with various neurological disorders.

Acute overdosage with barbiturates is manifested by CNS and respiratory depression which may progress to Cheyne- Stokes respiration, areflexia, constriction of the pupils to a slight degree (though in severe poisoning they may wshow paralytic dilation), oliguria, tachycardia, hypotension, lowered body temperature, and coma. Typical shock syndrome (apnea, circulatory collapse, respiratory arrest, and death) may occur.

In extreme overdose, all electrical activity in the brain may cease, in which case a ”flat” EEG normally equated with clinical death cannot be accepted. This effect is fully reversible unless hypoxic damage occurs. Consideration should be given to the possibility of barbiturate intoxication even in situations that appear to involve trauma.

Complications such as pneumonia, pulmonary edema, cardiac arrhythmias, congestive heart failure, and renal failure may occur. Uremia may increase CNS sensitivity to barbiturates if renal function is impaired. Differential diagnosis should include hypoglycemia, head trauma, cerebrovascular accidents, convulsive states, and diabetic coma.

1. Under the influence and appreciably impaired for purposes of driving a motor vehicle or performing tasks requiring alertness and unimpaired judgment and reaction time.

2. Sedated, therapeutic range, calm, relaxed, and easily aroused.

3. Comatose, difficult to arouse, significant depression of respiration.

4. Compatible with death in aged or ill persons or in presence of obstructed airway, other toxic agent, or exposure to cold.

5. Usual lethal level, the upper end of the range includes those who received some supportive treatment.

Treatment of overdosage is mainly supportive and consists of the following:

1. Maintenance of an adequate airway with assisted respiration and oxygen administration as necessary.

2. Monitoring of vital signs and fluid balance.

3. If the patient is conscious and has not lost the gag reflex, emesis may be induced with ipecac. Care should be taken to prevent pulmonary aspiration of vomitus. After completion of vomiting, 30 grams activated charcoal in a glass of water may be administered.

4. If emesis is contraindicated, gastric lavage may be performed with a cuffed endotracheal tube in place with the patient in the face down position. Activated charcoal may be left in the emptied stomach and a saline cathartic administered.

5. Fluid therapy and other standard treatment for shock, if needed.

6. If renal function is normal, forced diuresis may aid in the elimination of the barbiturate. Alkalinization of the urine increases renal excretion of some barbiturates, especially phenobarbital, also aprobarbital, and mephobarbital (which is metabolized to phenobarbital).

7. Although not recommended as a routine procedure, hemodialysis may be used in severe barbiturate intoxications or if the patient is anuric or in shock.

8. Patient should be rolled from side to side every 30 minutes.

9. Antibiotics should be given if pneumonia is suspected.

10. Appropriate nursing care to prevent hypostatic pneumonia, decubiti, aspiration and other complications of patients with altered states of consciousness.

Contraindications

Phenobarbital is contraindicated in patients with known phenobarbital sensitivity or a history of manifest or latent porphyria.

Phenobarbital is capable of producing all levels of CNS mood alteration, from excitation to mild sedation to hypnosis, and deep coma. Overdosage can produce death. In high enough therapeutic doses, Phenobarbital induces anesthesia. Phenobarbital depresses the sensory cortex, decreases motor activity, alters cerebellar function, and produces drowsiness, sedation, and hypnosis. Phenobarbital-induced sleep differs from physiological sleep. Sleep laboratory studies have demonstrated that Phenobarbital reduces the amount of time spent in the rapid eye movement (REM) phase of sleep or the dreaming stage. Also Stages III and IV sleep are decreased. Following abrupt cessation of Phenobarbital used regularly, patients may experience markedly increased dreaming, nightmares and/or insomnia. Therefore, withdrawal of a single therapeutic dose over 5 or 6 days has been recommended to lessen the REM rebound and disturbed sleep which contribute to drug withdrawal syndrome (for example, decrease the dose from 3 to 2 doses a day for 1 week).

Phenobarbital might be expected to lose its effectiveness for inducing and maintaining sleep after about 2 weeks.

Phenobarbital has little analgesic action at subanesthetic doses. Rather, in subanesthetic doses, this drug may increase the reaction to painful stimuli. All barbiturates exhibit anticonvulsant activity in anesthetic doses. However, of the drugs in this class, only phenobarbital, mephobarbital, and metharbital are effective as oral anticonvulsants in subhypnotic doses.

Phenobarbital is a respiratory depressant. The degree of respiratory depression is dependent upon the dose. With hypnotic doses, respiratory depression produced by Phenobarbital is similar to that which occurs during physiologic sleep with slight decrease in blood pressure and heart rate. Studies in laboratory animals have shown that Phenobarbital causes reduction in the tone and contractility of the uterus, ureters, and urinary bladder. However, concentrations of the drug required to produce this effect in humans are not reached with sedative-hypnotic doses.

Phenobarbital does not impair normal hepatic function but has been shown to induce liver microsomal enzymes, thus increasing and/or altering the metabolism of barbiturates and other drugs. (See DRUG INTERACTIONS.)

Pharmacokinetics

Phenobarbital is absorbed in varying degrees following oral, rectal or parenteral administration. The salts are more rapidly absorbed than are the acids. The rate of absorption is increased if the sodium salt is ingested as a dilute solution or taken on an empty stomach.

Duration of action, which is related to the rate at which phenobarbital is redistributed throughout the body varies among persons and in the same person from time to time. Long-acting phenobarbital has onset of action of 1 hour or longer and duration of actions of 10 to 12 hours.

No studies have demonstrated that the different routes of administration are equivalent with respect to bioavailability.

Phenobarbital is a weak acid that is absorbed and rapidly distributed to all tissues and fluids with high concentrations in the brain, liver, and kidneys. The more lipid soluble the drug is, the more rapidly it penetrates all tissues of the body.

Phenobarbital has the lowest lipid solubility, lowest plasma binding, lowest brain protein binding, the longest delay in onset of activity, and the longest duration of action in the barbiturate class.

Phenobarbital has a plasma half-life of 53 to 118 hours (mean: 79 hours). For children and newborns the plasma half-life is 60 to 180 hours (mean: 110 hours). (Half-life values were determined for newborn age being defined as 48 hours or less.)

Phenobarbital is metabolized primarily by the hepatic microsomal enzyme system, and the metabolic products are excreted in the urine, and less commonly, in the feces. Approximately 25 to 50 percent of a dose of phenobarbital is eliminated unchanged in the urine, whereas the amount of other barbiturates excreted unchanged in the urine is negligible. The excretion of unmetabolized barbiturate is one feature that distinguishes the long-acting category from those belonging to other categories which are almost entirely metabolized. The inactive metabolites of the barbiturates are excreted as conjugates of glucuronic acid.