Spironolactone and warfarin interaction

Area of Science:

• Pharmacology
• Clinical Medicine
• Drug Interactions

Background:

• Spironolactone is known to affect drug metabolism and levels.
• Previous animal studies suggested spironolactone may diminish the hypoprothrombinemic effect of oral anticoagulants.
• The mechanism of this interaction in humans required further investigation.

Purpose of the Study:

• To evaluate the effect of spironolactone on the hypoprothrombinemic effect of warfarin in humans.
• To determine if spironolactone alters warfarin levels or its anticoagulant activity.

Main Methods:

• Nine healthy subjects received single oral doses of warfarin (1.5 mg/kg) with and without daily oral spironolactone (200 mg).
• Blood samples were collected daily to measure 1-stage prothrombin activity, hematocrit, and warfarin concentration.
• Statistical analysis was performed to compare outcomes between the two treatment arms.

Main Results:

• A significant decrease in the hypoprothrombinemic effect of warfarin was observed when co-administered with spironolactone (p < 0.01).
• Venous hematocrit significantly increased during combined therapy (p < 0.001), indicating diuresis.
• No significant change in warfarin blood levels was detected (p > 0.5).

Conclusions:

• The interaction between warfarin and spironolactone in humans is primarily mediated by spironolactone-induced diuresis.
• Diuresis leads to a concentration of clotting factors, resulting in a decreased anticoagulant effect of warfarin.
• Spironolactone does not appear to significantly alter warfarin blood levels in this context.