Pharmacy Drug Interactions 3

Question 1

Rifampin is a potent inducer of CYP450 enzymes. When a patient on oral contraceptives starts rifampin, you should warn them about:

Accepted Answer

Decreased contraceptive efficacy and risk of unintended pregnancy

Answer

Rifampin induces CYP3A4, accelerating metabolism of estrogen/progestin in oral contraceptives, reducing their plasma levels and effectiveness.

Question 2

The combination of sildenafil and nitrates is contraindicated because it can cause:

Accepted Answer

Severe hypotension

Answer

Both nitrates and PDE5 inhibitors (sildenafil) decrease cGMP degradation/increase nitric oxide, causing additive vasodilation and potentially fatal hypotension.

Question 3

Which drug interaction occurs when phenytoin decreases the effect of doxycycline by increasing its metabolism?

Accepted Answer

CYP enzyme induction

Answer

Phenytoin is a CYP enzyme inducer; it increases the metabolism of doxycycline, reducing its half-life and therapeutic plasma levels.

Question 4

Lithium toxicity risk increases when a patient starts a thiazide diuretic because:

Accepted Answer

Thiazides cause sodium depletion, leading to compensatory lithium reabsorption

Answer

Thiazides cause sodium and water loss; the kidney compensates by reabsorbing more sodium — and lithium, which mimics sodium — raising lithium levels to potentially toxic concentrations.

Question 5

A patient taking digoxin begins amiodarone therapy. What adjustment is typically needed?

Accepted Answer

Decrease the digoxin dose by approximately 50%

Answer

Amiodarone inhibits P-glycoprotein and CYP3A4/CYP2D6, reducing digoxin clearance. The digoxin dose should be reduced ~50% to avoid toxicity.

Pharmacy Practice Test

Pharmacy Practice Test

Pharmacy Drug Interactions 3