Drug Interactions Flashcards

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

Answer: Decreased contraceptive efficacy and risk of unintended pregnancy

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

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

Answer: Severe hypotension

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

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

Answer: CYP enzyme induction

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

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

Answer: Thiazides cause sodium depletion, leading to compensatory lithium reabsorption

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.

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

Answer: Decrease the digoxin dose by approximately 50%

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

Taking an MAOI within 14 days of starting meperidine can cause which life-threatening reaction?

Answer: Serotonin syndrome / hyperpyrexic crisis

Meperidine inhibits serotonin reuptake; combined with MAOIs, this causes serotonin syndrome presenting as hyperpyrexia, rigidity, seizures, and cardiovascular instability.