NAPLEX Practice Question # 32

NAPLEX Examination.

Practice Question # 32.




Phenobarbital (Luminal)

 Mechanism of action.

Phenobarbital increases the seizure threshold by decreasing postsynaptic excitation by stimulating postsynaptic GABA-A receptor inhibitor responses as a CNS depressant.

 Administration and dosage

  • For adults, phenobarbital is administered orally at 90 to 300 mg daily (in three divided doses or as a single dose at bedtime).
  • Children typically receive 3 to 6 mg/kg daily in two divided doses. Adjustment is made as needed.

Adverse effects.

The physician should be notifi ed if any of the following adverse effects occur: sore throat, mouth sores, easy bruising or bleeding, and any signs of infection.

  • CNS eff ects include agitation, confusion, lethargy, and drowsiness. Patients should avoid alcohol and other CNS depressants.
  • Respiratory eff ects include hypoventilation and apnea.
  • Cardiovascular eff ects include bradycardia and hypotension.
  • GI eff ects include nausea, diarrhea, and constipation. If GI upset is experienced, phenobarbital should be taken with food.
  • Hematological eff ects include megaloblastic anemia aft er chronic use (a rare side effect).
  • Miscellaneous eff ects include osteomalacia and Stevens–Johnson syndrome, both of which are rare.

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