Warfarin

Question 1

Drug class?

Answer 1

Oral anticoagulant

Question 2

How does it work?

Answer 2

It is a vitamin K antagonist and it acts by inhibiting the formation of active clotting factors II, VII, IX and X.

Question 3

Indications?

Answer 3

• Prophylaxis of systemic embolism in patients with rheumatic heart disease and atrial fibrillation.
• Prophylaxis after insertion of prosthetic heart valves.
• Prophylaxis and treatment of venous thrombosis and pulmonary embolism.
• Transient attacks of cerebral ischaemia.

Question 4

Contra-indications?

Answer 4

• Haemorrhagic stroke.
• Clinically significant bleeding
• Within 72 hours of major surgery with risk of severe bleeding.
• Within 48 hours postpartum
• Pregnancy (first and third trimesters).
• Drugs where interactions may lead to a significantly increased risk of bleeding.

Question 5

Side effects?

Answer 5

Haemorrhage is most common undesirable effect reported.

Question 6

Possible interactions?

Answer 6

• Concomitant use of drugs used in the treatment or prophylaxis of thrombosis, or other drugs with adverse effects on haemostasis may increase the pharmacological effect of warfarin, increasing the risk of bleeding.
• Fibrinolytic drugs such as streptokinase and alteplase are contraindicated in patients receiving warfarin.
• Warfarin metabolised by Cytochrome P450 system. Drugs which compete as substrates or inhibit P450 system increase warfarin concentration (increased INR). Drugs which induce the P450 system decrease warfarin plasma concentration (decreased INR).