Major Drug Interactions

Question 1

Digoxin and amiodarone?

Answer 1

Digoxin levels increased - increased risk of toxicity. Digoxin dose needs to be halved.

(Amiodarone is a potent inhibitor)

Question 2

Is Ciclosporin an inhibitor or inducer?

Answer 2

Inhibitor of CYP3A4

Ciclosporin is also an inhibitor of P-glycoprotein multidrug efflux transporter

Question 3

Digoxin and Quinine?

Answer 3

Digoxin concentration increased - dose should be halved

Quinine increases QT prolongation and has been shown to cause AV conduction disturbances

Question 4

Sildenafil and Nitrates?

Answer 4

Sildenafil enhanced hypotension effect of nitrates. Avoid concomitant use

(Sildenafil also increases hypotension effect if given concomitantly with alpha blockers or nicorandil)

Question 5

Clonidine and B-blockers?

Answer 5

Increased risk of withdrawal hypertension.

B-blockers should be withdrawn slowly several days before clonidine

Question 6

Pimozide and amiodarone?

Answer 6

Increased risk of ventricular arrythmias

Pimozide has shown to increase QT interval

Question 7

NSAIDs and SSRIs?

Answer 7

Increased risk of bleeding

Question 8

NSAIDs and ciclosporin?

Answer 8

Increased risk of nephrotoxicity

Question 9

NSAIDs and coumarins?

Answer 9

Enhanced anticoagulant effect - increased risk of bleeding

Question 10

NSAIDs and lithium?

Answer 10

Possible reduced lithium excretion (increased risk of lithium toxicity)

Question 11

NSAIDs and methotrexate?

Answer 11

Possible reduced excretion of methotrexate (increased risk of methotrexate toxicity)

Question 12

NSAIDs and quinolones?

Answer 12

Possible increased risk of convulsions

Question 13

Methotrexate and trimethoprim?

Answer 13

increased risk of severe bone marrow depression and haematological toxicities

Question 14

Cytotoxics and clozapine?

Answer 14

Increased risk of agranulocytosis