Common drug interactions

Question 1

Warfarin and erthyromycin

Answer 1

Erythromycin is an enzyme inhibitor and will reduce P450 enzyme activity. This will increase warfarin levels, leading to an increased INR.

Question 2

Common inducers (PC BRAS)

Answer 2

Phenytoin, Carbamazepine, Barbituates, Rifampicin, Alcohol (chronic excess), Sulphonylureas

Question 3

Common inhibitors (AODEVICES)

Answer 3

Allopurinol, Omeprazole, Disulfiram, Erythromycin, Valproate, Isoniazid, Ciprofloxacin, Ethanol (acute intoxication), Sulphonamides

Question 4

Verapamil and beta-blockers

Answer 4

Risk of bradycardia (or asytole) and hypotension as both are rate-limiting drugs

Question 5

How is paracetamol metabolised?

Answer 5

By the liver in a process reliant on the antioxidant glutathione. In overdose, the liver stores of glutathione are depleted and an alternative pathway is used, creating toxic TAPQI.

Question 6

How is paracetamol overdose treated?

Answer 6

With NAC which replenishes glutathione stores.

Question 7

What is a normal INR?

What is the target INR for someone on warfarin therapy?

Answer 7

1

2.5

Question 8

What does a high/low INR mean?

Answer 8

Low means a short clotting time. High means a long clotting time.
INR is based on prothrombin time (PT).

Question 9

Antidepressant can cause…

Answer 9

Hyponatraemia

Question 10

What can all heparins cause?

Answer 10

Hyperkalaemia