PSA Drug Interactions 2

Question 1

A patient stabilized on warfarin begins rifampicin for tuberculosis. What is the most likely effect on the INR?

Accepted Answer

INR decreases due to CYP2C9 induction by rifampicin

Answer

Rifampicin is a potent CYP2C9 inducer that accelerates warfarin metabolism, substantially reducing its anticoagulant effect and lowering the INR.

Question 2

A patient on simvastatin 40 mg is prescribed clarithromycin for a respiratory infection. What is the primary risk?

Accepted Answer

Increased risk of myopathy or rhabdomyolysis due to elevated simvastatin levels

Answer

Clarithromycin potently inhibits CYP3A4, the primary enzyme metabolizing simvastatin, causing plasma accumulation and a significantly increased risk of muscle toxicity.

Question 3

A patient receiving standard-dose codeine reports no pain relief. Which CYP enzyme variant most likely explains this pharmacogenomic interaction?

Accepted Answer

CYP2D6 poor metabolizer preventing conversion of codeine to morphine

Answer

CYP2D6 converts codeine to its active analgesic metabolite morphine; poor metabolizers (7–10% of Caucasians) derive minimal analgesia from codeine.

Question 4

Fluoxetine (a potent CYP2D6 inhibitor) is started in a patient already taking tramadol. What is the most important risk?

Accepted Answer

Tramadol accumulation and serotonin syndrome risk due to CYP2D6 inhibition

Answer

Fluoxetine inhibits CYP2D6, which metabolizes tramadol to its active form; inhibition elevates parent tramadol levels and impairs serotonin reuptake clearance, raising serotonin syndrome risk.

Question 5

A patient on combined oral contraceptives starts carbamazepine for epilepsy. What is the pharmacokinetic mechanism of concern?

Accepted Answer

CYP3A4 induction by carbamazepine reduces estrogen and progestogen plasma levels

Answer

Carbamazepine is a potent CYP3A4 inducer that accelerates hepatic metabolism of ethinylestradiol and progestogens, reducing contraceptive hormone levels and risking contraceptive failure.

(PSA) Prescribing Safety Assessment Practice Test

(PSA) Prescribing Safety Assessment Practice Test

PSA Drug Interactions 2