phenobarbital (Rx)

Frequency Not Defined

IV

• Respiratory depression

Common

• Ataxia
• Dizziness
• Drowsiness
• Dysarthria
• Fatigue
• Headache
• Irritability
• Nystagmus
• Paresthesia restlessness
• Vertigo
• Geriatric patients: Excitement, confusion, depression
• Pediatric patients: Paradoxical excitement/hyperactivity

Less Common

• Mental dullness
• Constipation
• Diarrhea
• Nausea
• Vomiting
• Megaloblastic (folate-deficiency) anemia

Uncommon

• Rash
• Hypocalcemia
• Hepatotoxicity

Rare

• Stevens-Johnson syndrome
• Rickets
• Osteomalacia

Contraindications

Hypersensitivity

Porphyria

Intra-arterial or subcutaneous administration

Severe hepatic impairment

Dyspnea or airway obstruction

History of sedative hyponotic addiction

Nephritic patients (large doses)

Pregnancy/lactation

Cautions

Commercial injection is highly alkaline and may cause tissue necrosis if given SC or if it extravasates (if happens, treat with application of moist heat and injection of 0.5% procaine)

May render oral contraceptives ineffective

Paradoxical responses may occur, especially in pediatric patients and patients experiencing acute or chronic pain

May cause CNS depression, causing physical and mental impairment; caution patient about performing tasks that require mental alertness

May cause respiratory depression, especially with intravenous administration; use caution in patients with respiratory disease and status asthmaticus

Withdraw therapy gradually; abrupt discontinuation, may increase seizure frequency

Use caution in patients with severe anemia, patients experiencing depression, cardiac disease, hemodynamically unstable patients, hyperthyroidism, renal impairment, hypoadrenalism, and in patients with diabetes

May mask important symptoms when used in patients with acute or chronic pain; use caution

Pregnancy category: D

Lactation: Do not nurse

Pregnancy Categories

A: Generally acceptable. Controlled studies in pregnant women show no evidence of fetal risk.

B: May be acceptable. Either animal studies show no risk but human studies not available or animal studies showed minor risks and human studies done and showed no risk.

C: Use with caution if benefits outweigh risks. Animal studies show risk and human studies not available or neither animal nor human studies done.

D: Use in LIFE-THREATENING emergencies when no safer drug available. Positive evidence of human fetal risk.

X: Do not use in pregnancy. Risks involved outweigh potential benefits. Safer alternatives exist.

NA: Information not available.

Mechanism of Action

Depresses sensory and motor cortex, cerebellum

Antiseizure activity occurs primarily where GABA mediates neurotransmission

Hypnotic effects of barbiturates result from activity at GABA receptor in the polysynaptic midbrain reticular formation (controls CNS arousal)

Off-label use for hyperbilirubinemia: Phenobarbital induces glucuronyl transferase and hepatic bilirubin-binding Y-protein to lower serum bilirubin concentrations

Absorption

Bioavailability: 70-90%

Onset: 5 min (IV)

Duration: 4-6 hr (IV/IM)

Peak plasma time: 8-12 hr

Therapeutic plasma concentration: 10-40 mcg/mL; may require 3-4 weeks of treatment to achieve therapeutic levels

Distribution

Protein bound: 20-45%

Metabolism

Metabolized by hepatic oxidative hydroxylation

Metabolites: Inactive

Enzymes induced: CYP1A2, CYP2B6, CYP2C19, CYP2C9/10, CYP3A4

Elimination

Half-life: 50-140 hr

Excretion: Urine (major)

IV Incompatibilities

Additive: Chlorpromazine, ephedrine, hydralazine, hydrocortisone Na-succinate, hydroxyzine, insulin, levorphanol, meperidine, morphine, norepinephrine, pentazocine, procaine, prochlorperazine, promazine, promethazine, streptomycin, vancomycin

Syringe: Hydromorphone, ranitidine, sufentanil

Y-site: Amphotericin B cholSO4

IV Compatibilities

Solution: Dextrose Ringer, dextrose LR, dextrose saline, Ringer, LR, NS, ½ NS, Na-lactate 1/6 M, D5W, D10W

Additive: Amikacin, aminophylline, CaCl2, Ca-gluconate, colistimethate, dimenhydrinate, meropenem, polymyxin B, thiopental, verapamil

Syringe: Caffeine, heparin

Y-site: Doxapram, enalaprilat, fentanyl, fosphenytoin gatifloxacin, hydromorphone (may be incompatible at high concentrations of both), levofloxacin, linezolid, meropenem, methadone, morphine SO4, propofol, sufentanil

IV Administration

Slow injection at <60 mg/min

May be given IM into large muscle but not SC

Commercial injection is highly alkaline and can cause local tissue necrosis

Stability

Store intact vials at room temperature, protected from light