PEBC Drug Interactions and Adverse Drug Reactions

Question 1

A patient on warfarin begins taking clarithromycin for a respiratory infection. Which mechanism best explains the increased bleeding risk?

Accepted Answer

Clarithromycin inhibits CYP3A4 and CYP2C9, reducing warfarin metabolism and increasing its plasma levels

Answer

Clarithromycin is a potent inhibitor of CYP3A4 and also inhibits CYP2C9, which metabolizes warfarin. Inhibition of these enzymes reduces warfarin clearance, elevating INR and increasing hemorrhage risk. INR should be monitored closely.

Question 2

Which combination represents a potentially life-threatening pharmacodynamic drug interaction requiring strict avoidance?

Accepted Answer

Serotonergic antidepressant (SSRI) and tramadol

Answer

Combining an SSRI with tramadol (which also inhibits serotonin reuptake and is a weak serotonin agonist) can precipitate serotonin syndrome — a potentially fatal drug reaction characterized by hyperthermia, neuromuscular abnormalities, and autonomic instability.

Question 3

A patient taking a monoamine oxidase inhibitor (MAOI) for depression consumes a meal rich in tyramine (e.g., aged cheese, cured meats). What adverse reaction is most likely?

Accepted Answer

Hypertensive crisis due to uninhibited tyramine-induced norepinephrine release

Answer

MAOIs inhibit monoamine oxidase in the gut and liver that normally degrades tyramine. When tyramine is absorbed intact, it triggers massive norepinephrine release from sympathetic nerve terminals, causing potentially fatal hypertensive crisis — the classic MAO-tyramine food interaction.

Question 4

Which of the following is a Type A (augmented) adverse drug reaction?

Accepted Answer

Bradycardia and hypotension from an excessive dose of metoprolol

Answer

Type A adverse drug reactions are dose-dependent, predictable extensions of the drug's known pharmacological effect. Bradycardia from beta-blocker overdose is a predictable, dose-related ADR. Types B (bizarre) reactions like anaphylaxis and SJS are idiosyncratic and not related to normal pharmacology.

Question 5

A patient is started on simvastatin 80 mg and also takes gemfibrozil for hypertriglyceridemia. Which serious drug interaction risk should prompt the pharmacist to intervene?

Accepted Answer

Increased risk of myopathy and rhabdomyolysis due to elevated simvastatin plasma concentrations

Answer

Gemfibrozil inhibits OATP1B1 and CYP2C8, significantly increasing simvastatin plasma concentrations and dramatically raising the risk of myopathy and rhabdomyolysis. This combination is generally contraindicated; if a fibrate is needed, fenofibrate is preferred over gemfibrozil with statins.

PEBC - Pharmacy Examining Board of Canada Practice Test

PEBC - Pharmacy Examining Board of Canada Practice Test

PEBC Drug Interactions and Adverse Drug Reactions