Phenobarbital MCQs With Answer

Phenobarbital MCQs With Answer help B. Pharm students master phenobarbital pharmacology, clinical uses, dosing, adverse effects, and drug interactions through targeted practice. This focused set emphasizes mechanism of action, pharmacokinetics, therapeutic drug monitoring, toxicity management, and special population considerations such as neonates, pregnancy, and the elderly. Questions reinforce concepts like GABA-A modulation, CYP induction, therapeutic ranges, contraindications (porphyria), and monitoring plasma levels. Case-oriented items explore formulations, IV loading regimens, drug interactions with warfarin and oral contraceptives, and overdose management. Ideal for revision, classroom quizzes, and self-assessment. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. Which best describes the primary mechanism of action of phenobarbital?

• Blocks sodium channels to prevent action potential generation
• Enhances GABA-A receptor-mediated chloride influx by increasing channel opening duration
• Inhibits NMDA receptors to reduce excitatory neurotransmission
• Increases serotonin release in the synaptic cleft

Correct Answer: Enhances GABA-A receptor-mediated chloride influx by increasing channel opening duration

Q2. Phenobarbital is classified pharmacologically as:

• A benzodiazepine
• A barbiturate
• A hydantoin
• A succinimide

Correct Answer: A barbiturate

Q3. The usual therapeutic plasma concentration range for phenobarbital in adults is:

• 1–5 mcg/mL
• 5–10 mcg/mL
• 15–40 mcg/mL
• 50–100 mcg/mL

Correct Answer: 15–40 mcg/mL

Q4. Phenobarbital is most effective clinically for which seizure types?

• Absence seizures only
• Generalized tonic-clonic and partial seizures
• Myoclonic seizures primarily
• Febrile seizures exclusively

Correct Answer: Generalized tonic-clonic and partial seizures

Q5. The typical intravenous loading dose of phenobarbital for acute seizure control is approximately:

• 1–2 mg/kg
• 5–10 mg/kg
• 15–20 mg/kg
• 50 mg/kg

Correct Answer: 15–20 mg/kg

Q6. Which statement about phenobarbital pharmacokinetics is correct?

• It has a very short half-life of 2–4 hours
• It is primarily excreted unchanged in bile
• It has a long half-life (often days) and is metabolized by hepatic enzymes
• It is not metabolized and is eliminated by pulmonary exhalation

Correct Answer: It has a long half-life (often days) and is metabolized by hepatic enzymes

Q7. Phenobarbital induces which of the following hepatic enzymes, causing many drug interactions?

• CYP450 enzymes (e.g., CYP3A4)
• CYP2E1 only
• Monoamine oxidase (MAO)
• Carbonic anhydrase

Correct Answer: CYP450 enzymes (e.g., CYP3A4)

Q8. Which adverse effect is most commonly associated with chronic phenobarbital therapy?

• Hypersalivation and hyperactivity
• Persistent cough and bronchospasm
• Sedation, cognitive impairment, and dependence
• Severe hypertension

Correct Answer: Sedation, cognitive impairment, and dependence

Q9. Phenobarbital is contraindicated in patients with which metabolic disorder?

• Phenylketonuria
• Acute intermittent porphyria
• G6PD deficiency
• Lactose intolerance

Correct Answer: Acute intermittent porphyria

Q10. Which of the following best describes phenobarbital’s effect on oral contraceptives?

• It increases contraceptive efficacy by reducing estrogen metabolism
• It has no effect on oral contraceptives
• It reduces contraceptive efficacy by inducing estrogen metabolism
• It converts combined pills into progestin-only pills

Correct Answer: It reduces contraceptive efficacy by inducing estrogen metabolism

Q11. In neonates, phenobarbital dosing considerations differ because:

• Neonates have increased hepatic metabolism leading to shorter half-life
• Neonates have reduced protein binding and immature hepatic enzymes increasing free drug
• Phenobarbital is ineffective in neonates due to receptor immaturity
• Neonates rapidly excrete phenobarbital in bile

Correct Answer: Neonates have reduced protein binding and immature hepatic enzymes increasing free drug

Q12. Which monitoring parameter is most important during chronic phenobarbital therapy?

• Therapeutic drug plasma concentration (TDM)
• Serum potassium levels only
• Skin prick testing monthly
• Daily arterial blood gases

Correct Answer: Therapeutic drug plasma concentration (TDM)

Q13. Which management is most appropriate for severe phenobarbital overdose?

• Administer beta blockers immediately
• Supportive care with airway protection and consider hemodialysis
• Give high-dose aspirin to enhance elimination
• Immediate liver transplant

Correct Answer: Supportive care with airway protection and consider hemodialysis

Q14. Phenobarbital’s effect on the GABA-A receptor is most similar to which class of drugs?

• Benzodiazepines (e.g., diazepam) in enhancing GABAergic inhibition
• Opioids (e.g., morphine) in mu receptor activation
• SSRIs in increasing serotonin levels
• Beta blockers in blocking adrenergic receptors

Correct Answer: Benzodiazepines (e.g., diazepam) in enhancing GABAergic inhibition

Q15. Which is a recognized hematologic adverse effect of long-term phenobarbital therapy?

• Leukopenia and megaloblastic anemia
• Thrombocytosis only
• Polycythemia vera
• Hemophilia A

Correct Answer: Leukopenia and megaloblastic anemia

Q16. In patients taking warfarin, coadministration of phenobarbital typically causes:

• Increased warfarin effect and bleeding risk
• No interaction with warfarin
• Decreased warfarin effect due to induction of warfarin metabolism
• Immediate antagonism of warfarin binding sites

Correct Answer: Decreased warfarin effect due to induction of warfarin metabolism

Q17. Which formulation of phenobarbital is commonly used for rapid control of seizures in hospital settings?

• Oral enteric-coated tablet
• Intramuscular oil solution
• Intravenous phenobarbital sodium injection
• Transdermal patch

Correct Answer: Intravenous phenobarbital sodium injection

Q18. Phenobarbital’s protein binding is approximately:

• Less than 5%
• About 45% (moderate binding)
• Over 99% (extremely high)
• Zero; it remains entirely unbound

Correct Answer: About 45% (moderate binding)

Q19. Which statement about phenobarbital in pregnancy is true?

• It is completely safe and recommended for all pregnant women
• It is associated with teratogenic risk and neonatal withdrawal, so use only if necessary
• It prevents congenital malformations associated with seizures
• It increases folate levels and reduces birth defects

Correct Answer: It is associated with teratogenic risk and neonatal withdrawal, so use only if necessary

Q20. Which lab test may be useful to monitor phenobarbital toxicity or dosing adequacy?

• Serum phenobarbital concentration
• Urine ketone levels
• Serum amylase only
• CSF glucose

Correct Answer: Serum phenobarbital concentration

Q21. Which drug interaction is most likely when phenobarbital is started in a patient on phenytoin?

• Phenobarbital will increase phenytoin levels dramatically
• Phenobarbital may induce hepatic enzymes and lower phenytoin levels
• There is no interaction between phenobarbital and phenytoin
• They form inactive complexes in the gut reducing absorption

Correct Answer: Phenobarbital may induce hepatic enzymes and lower phenytoin levels

Q22. The onset of action after intravenous administration of phenobarbital is approximately:

• Immediate to a few minutes
• 12–24 hours
• 3–4 days
• One week

Correct Answer: Immediate to a few minutes

Q23. Which sign is characteristic of phenobarbital overdose?

• Marked agitation and hyperreflexia
• Progressive CNS depression and respiratory depression
• Severe hypertension and tachycardia
• Profound muscle rigidity

Correct Answer: Progressive CNS depression and respiratory depression

Q24. Phenobarbital induces its own metabolism. This phenomenon is called:

• Autoinduction
• First-pass effect
• Zero-order kinetics
• Michaelis-Menten saturation

Correct Answer: Autoinduction

Q25. Which vitamin deficiency may be exacerbated by long-term phenobarbital therapy?

• Vitamin C deficiency
• Vitamin D and folate deficiency due to enzyme induction
• Vitamin B12 exclusively
• No vitamin deficiencies are associated

Correct Answer: Vitamin D and folate deficiency due to enzyme induction

Q26. When converting from intravenous to oral phenobarbital, the pharmacist should remember that:

• Bioavailability of oral phenobarbital is poor and unpredictable
• Oral phenobarbital has excellent bioavailability, often near 100%
• Oral dosing must be doubled due to first-pass destruction
• Oral route is contraindicated after IV use

Correct Answer: Oral phenobarbital has excellent bioavailability, often near 100%

Q27. Which is a common withdrawal symptom after abrupt discontinuation of chronic phenobarbital therapy?

• Insomnia and increased seizure frequency
• Exuberant hair growth
• Increased salivation and lacrimation only
• Improved cognitive function immediately

Correct Answer: Insomnia and increased seizure frequency

Q28. Phenobarbital is sometimes used as a first-line anticonvulsant in which population?

• Young adults with absence seizures
• Neonates with neonatal seizures
• Adolescents with migraine
• Pregnant women preferentially over lamotrigine

Correct Answer: Neonates with neonatal seizures

Q29. Which metabolic pathway is primarily responsible for phenobarbital clearance?

• Renal glomerular filtration of unchanged drug only
• Hepatic microsomal oxidation (CYP-mediated) followed by conjugation
• Metabolism by cholinesterases in plasma
• Degradation by gut flora only

Correct Answer: Hepatic microsomal oxidation (CYP-mediated) followed by conjugation

Q30. In elderly patients, phenobarbital dosing requires caution because:

• Elderly patients clear the drug faster and need higher doses
• They are more sensitive to CNS depression and drug accumulation due to reduced clearance
• Phenobarbital causes increase in bone density in elderly
• It reliably improves cognition in the elderly

Correct Answer: They are more sensitive to CNS depression and drug accumulation due to reduced clearance

Q31. Which of the following describes phenobarbital’s effect on cytochrome P450 substrates?

• It acts as a competitive inhibitor of P450 enzymes
• It induces P450 enzymes, decreasing plasma levels of many drugs
• It has no effect on drug-metabolizing enzymes
• It irreversibly inactivates all P450 enzymes

Correct Answer: It induces P450 enzymes, decreasing plasma levels of many drugs

Q32. Which adverse effect warrants immediate discontinuation and evaluation when on phenobarbital?

• Mild transient headache
• Severe rash suggesting Stevens-Johnson syndrome
• Occasional dreaming
• Temporary mild constipation

Correct Answer: Severe rash suggesting Stevens-Johnson syndrome

Q33. A patient on chronic phenobarbital therapy develops signs of bone demineralization. Which mechanism explains this?

• Phenobarbital directly deposits calcium in kidneys
• Enzyme induction increases vitamin D catabolism leading to reduced bone mineralization
• Phenobarbital stimulates osteoblasts excessively
• It causes hypervitaminosis D

Correct Answer: Enzyme induction increases vitamin D catabolism leading to reduced bone mineralization

Q34. Phenobarbital’s classification in pregnancy by older FDA categories was:

• Category A: safe in pregnancy
• Category B: no risk in animals but none in humans
• Category D: evidence of human fetal risk but benefits may warrant use
• Category X: contraindicated in pregnancy

Correct Answer: Category D: evidence of human fetal risk but benefits may warrant use

Q35. Which of the following drugs’ levels are likely to decrease when co-administered with phenobarbital?

• Drugs metabolized by CYP450 such as oral contraceptives and warfarin
• Drugs eliminated solely by renal excretion without metabolism
• Drugs given IV as monoclonal antibodies
• Insulin levels in the bloodstream

Correct Answer: Drugs metabolized by CYP450 such as oral contraceptives and warfarin

Q36. Which clinical scenario most appropriately indicates phenobarbital use?

• Intermittent migraine prophylaxis in adults
• Long-term management of neonatal seizures and some generalized epilepsies
• Acute myocardial infarction pain control
• Short-term therapy for bacterial meningitis

Correct Answer: Long-term management of neonatal seizures and some generalized epilepsies

Q37. Which statement regarding phenobarbital tolerance is correct?

• Tolerance never develops to any effects of phenobarbital
• Tolerance may develop to sedative effects but less so to anticonvulsant effects
• Tolerance develops equally to anticonvulsant and sedative effects within 24 hours
• Tolerance enhances drug potency over time

Correct Answer: Tolerance may develop to sedative effects but less so to anticonvulsant effects

Q38. Which monitoring is advised when initiating phenobarbital with other hepatically metabolized drugs?

• No monitoring is required
• Monitor plasma concentrations of co-administered drugs and liver function tests
• Monitor only serum sodium
• Periodic electrocardiograms only

Correct Answer: Monitor plasma concentrations of co-administered drugs and liver function tests

Q39. The use of activated charcoal in phenobarbital overdose is useful because:

• Phenobarbital is poorly absorbed from the gut so charcoal has no role
• Charcoal can adsorb phenobarbital and reduce further absorption if given early
• Charcoal directly neutralizes phenobarbital toxicity in bloodstream
• Charcoal increases renal excretion of phenobarbital

Correct Answer: Charcoal can adsorb phenobarbital and reduce further absorption if given early

Q40. Which electroencephalographic (EEG) effect might be seen with therapeutic phenobarbital levels?

• Marked epileptiform discharges in all cases
• Generalized slowing reflecting sedative effect
• EEG becomes isoelectric in therapeutic doses
• EEG shows rapid beta activity exclusively

Correct Answer: Generalized slowing reflecting sedative effect

Q41. In patients with renal failure, phenobarbital dosing:

• Never needs adjustment because eliminated only by liver
• May require adjustment since metabolites and some unchanged drug are renally excreted and hemodialysis can remove drug
• Should be stopped completely due to immediate toxicity
• Is increased because renal failure enhances clearance

Correct Answer: May require adjustment since metabolites and some unchanged drug are renally excreted and hemodialysis can remove drug

Q42. Which neurotransmitter system is directly potentiated by phenobarbital leading to anticonvulsant action?

• Glutamatergic NMDA receptor system
• GABAergic inhibitory system
• Dopaminergic reward pathway
• Cholinergic nicotinic receptors

Correct Answer: GABAergic inhibitory system

Q43. Which statement about phenobarbital dosing frequency is correct given its pharmacokinetics?

• Requires multiple daily dosing due to very short half-life
• Often dosed once daily because of long half-life
• Must be given every hour to maintain levels
• Given only weekly due to depot effect

Correct Answer: Often dosed once daily because of long half-life

Q44. A patient on phenobarbital reports increased anxiety and irritability after dose reduction. This is most consistent with:

• Improved therapeutic response
• Withdrawal or rebound phenomenon
• Allergic reaction to phenobarbital
• Onset of major depressive disorder

Correct Answer: Withdrawal or rebound phenomenon

Q45. Which clinical lab abnormality may be associated with long-term phenobarbital therapy?

• Elevated hepatic transaminases due to enzyme induction or hepatotoxicity
• Marked hyperglycemia in all patients
• Hyperthyroidism universally
• Constantly elevated serum bicarbonate only

Correct Answer: Elevated hepatic transaminases due to enzyme induction or hepatotoxicity

Q46. Combining phenobarbital with which of these drugs increases risk of respiratory depression?

• Nonsteroidal anti-inflammatory drugs (NSAIDs)
• Benzodiazepines and opioids
• Antacids
• Topical corticosteroids

Correct Answer: Benzodiazepines and opioids

Q47. Which clinical use is NOT appropriate for phenobarbital?

• Management of certain epilepsies
• Short-term treatment of status epilepticus when benzodiazepines fail
• Routine first-line therapy for absence seizures in children
• Sedation in some clinical settings

Correct Answer: Routine first-line therapy for absence seizures in children

Q48. The pharmacist counseling point about alcohol consumption while taking phenobarbital should advise that:

• Alcohol has no interaction and is safe
• Concurrent alcohol increases CNS depression and is dangerous
• Alcohol increases phenobarbital clearance improving seizure control
• Alcohol prevents phenobarbital absorption

Correct Answer: Concurrent alcohol increases CNS depression and is dangerous

Q49. Which effect does phenobarbital have on neonatal bilirubin risk when used in pregnant women?

• It increases bilirubin binding in neonates, reducing risk
• It induces enzymes that may lower fetal bilirubin but risks of teratogenicity remain
• It has no impact on neonatal bilirubin whatsoever
• It always causes neonatal jaundice independent of dose

Correct Answer: It induces enzymes that may lower fetal bilirubin but risks of teratogenicity remain

Q50. When counseling a patient prescribed phenobarbital, which advice is MOST important?

• Stop the drug abruptly if you feel drowsy
• Be aware of sedation, avoid driving, and do not discontinue abruptly—follow monitoring recommendations
• You can take double doses if you miss one dose
• No need for any lab monitoring during therapy

Correct Answer: Be aware of sedation, avoid driving, and do not discontinue abruptly—follow monitoring recommendations

G S Sachin

I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.

Mail- Sachin@pharmacyfreak.com