Therapeutic incompatibilities with examples MCQs With Answer

Introduction: Therapeutic incompatibility refers to harmful or ineffective outcomes when drugs with opposing, excessive, or undesirable pharmacological actions are prescribed together. For B. Pharm students, understanding prescription incompatibilities—therapeutic, pharmacodynamic, pharmacokinetic, and clinical contraindications—is crucial for patient safety and rational pharmacotherapy. Key concepts include antagonism (e.g., beta-blocker with beta-agonist), additive/synergistic toxicity (e.g., benzodiazepine with opioids), enzyme induction/inhibition (e.g., rifampin with oral contraceptives), and absorption interactions (e.g., antacids with tetracycline). Recognizing high-risk pairs, populations (elderly, polypharmacy), and preventive strategies (medication review, interaction checkers, counseling) supports evidence-based dispensing and prescription screening. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. What best defines therapeutic incompatibility in a prescription?

• An undesirable pharmacologic outcome due to drug–drug interaction (antagonism or excessive synergism)
• Physical changes like precipitation when drugs are mixed
• Decomposition of drug due to chemical reaction
• Packaging-related product instability

Correct Answer: An undesirable pharmacologic outcome due to drug–drug interaction (antagonism or excessive synergism)

Q2. Which pair exemplifies pharmacological antagonism leading to therapeutic incompatibility?

• Propranolol with salbutamol in an asthmatic patient
• Penicillin with probenecid for syphilis
• Warfarin with NSAIDs for pain
• Iron with vitamin C

Correct Answer: Propranolol with salbutamol in an asthmatic patient

Q3. Which combination risks profound hypotension due to excessive vasodilation?

• Glyceryl trinitrate with sildenafil
• Atorvastatin with ezetimibe
• Metformin with insulin
• Furosemide with spironolactone

Correct Answer: Glyceryl trinitrate with sildenafil

Q4. Which scenario is NOT a therapeutic incompatibility?

• Precipitation of calcium phosphate in an IV admixture
• MAOI with tyramine-rich foods causing hypertensive crisis
• Rifampin reducing effectiveness of oral contraceptives
• Benzodiazepine with opioid causing respiratory depression

Correct Answer: Precipitation of calcium phosphate in an IV admixture

Q5. Rifampin co-administered with combined oral contraceptives leads to:

• Therapeutic failure due to enzyme induction
• Enhanced efficacy due to enzyme inhibition
• Physical incompatibility in the GI tract
• No clinically relevant interaction

Correct Answer: Therapeutic failure due to enzyme induction

Q6. Which pair shows dangerous additive CNS depression?

• Diazepam with morphine
• Atenolol with hydrochlorothiazide
• Metformin with sitagliptin
• Loratadine with montelukast

Correct Answer: Diazepam with morphine

Q7. Which antibiotic combination may reduce bactericidal efficacy in serious infections?

• Tetracycline with penicillin
• Amoxicillin with clavulanic acid
• Vancomycin with gentamicin (for endocarditis)
• Piperacillin with tazobactam

Correct Answer: Tetracycline with penicillin

Q8. On detecting a potentially incompatible prescription, the pharmacist’s first action should be:

• Contact the prescriber to clarify and optimize therapy
• Dispense as written and document later
• Substitute drugs without informing anyone
• Advise the patient to skip one drug arbitrarily

Correct Answer: Contact the prescriber to clarify and optimize therapy

Q9. Which population is at highest risk for clinically significant therapeutic incompatibilities?

• Elderly patients on polypharmacy
• Healthy adults on one medication
• Adolescents with seasonal allergies
• Postoperative patients on IV fluids only

Correct Answer: Elderly patients on polypharmacy

Q10. Which is an example of therapeutic duplication leading to increased toxicity risk?

• Concurrent use of two NSAIDs (e.g., ibuprofen and diclofenac)
• ACE inhibitor with thiazide diuretic
• Metformin with acarbose
• Aspirin with clopidogrel after stent

Correct Answer: Concurrent use of two NSAIDs (e.g., ibuprofen and diclofenac)

Q11. MAOI (e.g., phenelzine) with tyramine-rich foods may cause:

• Hypertensive crisis due to excess catecholamines
• Severe hypotension due to vasodilation
• Serotonin syndrome
• Hypoglycemia

Correct Answer: Hypertensive crisis due to excess catecholamines

Q12. Which pair increases the risk of hyperkalemia?

• ACE inhibitor with spironolactone
• Loop diuretic with digoxin
• Statin with fibrate
• Warfarin with vitamin K

Correct Answer: ACE inhibitor with spironolactone

Q13. Which is a pharmaceutical (not therapeutic) incompatibility?

• Color change and loss of potency due to oxidation in solution
• Serotonin syndrome with SSRI and linezolid
• Bleeding with warfarin and metronidazole
• Bronchospasm with nonselective beta-blocker in asthma

Correct Answer: Color change and loss of potency due to oxidation in solution

Q14. Which pair represents direct pharmacologic antagonism if co-prescribed unintentionally?

• Morphine with naloxone
• Insulin with metformin
• Amoxicillin with clavulanate
• Heparin with protamine during surgery

Correct Answer: Morphine with naloxone

Q15. Beta-blockers in insulin-treated diabetics may:

• Mask hypoglycemia and prolong recovery
• Prevent hypoglycemia via glycogenolysis
• Increase insulin secretion markedly
• Have no effect on glucose awareness

Correct Answer: Mask hypoglycemia and prolong recovery

Q16. Which combination increases INR and bleeding risk via enzyme inhibition?

• Warfarin with metronidazole
• Warfarin with rifampin
• Warfarin with carbamazepine
• Warfarin with cholestyramine

Correct Answer: Warfarin with metronidazole

Q17. Lithium toxicity is most likely with concurrent:

• Thiazide diuretics
• Calcium channel blockers
• Proton pump inhibitors
• H2 receptor antagonists

Correct Answer: Thiazide diuretics

Q18. Linezolid with SSRIs can precipitate:

• Serotonin syndrome
• Hypokalemia
• QT shortening
• Pulmonary fibrosis

Correct Answer: Serotonin syndrome

Q19. Combining metoprolol with verapamil primarily risks:

• Bradycardia and AV block
• Seizures
• Nephrotoxicity
• Photosensitivity

Correct Answer: Bradycardia and AV block

Q20. Which interaction reduces antifungal efficacy via decreased absorption?

• Antacids with ketoconazole
• Probenecid with penicillin
• Fluconazole with warfarin
• Food with itraconazole capsules

Correct Answer: Antacids with ketoconazole

Q21. Clarithromycin co-administered with digoxin may cause:

• Elevated digoxin levels via P-gp inhibition
• Reduced digoxin levels via induction
• No change in digoxin exposure
• Enhanced renal clearance of digoxin

Correct Answer: Elevated digoxin levels via P-gp inhibition

Q22. A key strategy to prevent therapeutic incompatibility at the pharmacy is:

• Using up-to-date drug interaction checkers for every new prescription
• Relying only on memory of common interactions
• Dispensing first, reviewing later
• Advising patients to self-adjust doses

Correct Answer: Using up-to-date drug interaction checkers for every new prescription

Q23. Fluoxetine with codeine often leads to:

• Reduced analgesia due to CYP2D6 inhibition
• Enhanced analgesia due to active metabolite formation
• Respiratory depression due to additive effects
• No clinically relevant interaction

Correct Answer: Reduced analgesia due to CYP2D6 inhibition

Q24. A patient on warfarin is prescribed trimethoprim–sulfamethoxazole. The best action is:

• Alert prescriber; consider alternative antibiotic or closer INR monitoring
• Dispense both without comment
• Stop warfarin abruptly
• Double the warfarin dose

Correct Answer: Alert prescriber; consider alternative antibiotic or closer INR monitoring

Q25. In anaphylaxis, which prior medication may blunt epinephrine response?

• Propranolol
• Amlodipine
• Lisinopril
• Hydrochlorothiazide

Correct Answer: Propranolol

Q26. Live vaccines are therapeutically incompatible with:

• High-dose systemic corticosteroids
• Antacids
• Alpha-1 blockers
• Selective COX-2 inhibitors

Correct Answer: High-dose systemic corticosteroids

Q27. Which combination increases risk of seizures and serotonin syndrome?

• Tramadol with MAOIs
• Gabapentin with acetaminophen
• Loratadine with pseudoephedrine
• Omeprazole with amoxicillin

Correct Answer: Tramadol with MAOIs

Q28. Co-administration of PDE5 inhibitors with alpha-1 blockers primarily risks:

• Symptomatic hypotension
• Nephrolithiasis
• Hypothyroidism
• Rhabdomyolysis

Correct Answer: Symptomatic hypotension

Q29. Methotrexate toxicity is increased by concurrent:

• NSAIDs (at high doses) impairing renal clearance
• Vitamin B12 supplementation
• Folic acid co-therapy
• Proton pump inhibitors improving clearance

Correct Answer: NSAIDs (at high doses) impairing renal clearance

Q30. Which pair is a rational synergistic combination, not a therapeutic incompatibility?

• Amoxicillin with clavulanic acid
• Sildenafil with nitrates
• Warfarin with metronidazole
• Propranolol with salbutamol

Correct Answer: Amoxicillin with clavulanic acid

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