Blood has to clot to stop bleeding, but clots that form where they are not needed can kill. An anticoagulant slows the clotting process, reducing the risk of stroke, deep vein thrombosis, and pulmonary embolism. These drugs do not dissolve existing clots. They prevent new ones from forming and keep existing ones from growing.

The classic agent is warfarin, a vitamin K antagonist that has been in use since the 1950s. It requires regular blood tests to check the INR, a measure of clotting time, because the dose window is narrow. Food and other drugs interact with it constantly. Newer direct oral anticoagulants like apixaban and rivaroxaban work faster, need less monitoring, and have largely replaced warfarin for many indications, though warfarin still holds ground for mechanical heart valves.

Key points for anyone on these drugs:

• Bleeding risk. Minor cuts take longer to stop. Serious bleeding is the main concern.
• Reversal agents. Vitamin K for warfarin, specific antidotes for the newer drugs.
• Interactions. NSAIDs, some antibiotics, and certain foods change the effect.
• Monitoring. Warfarin needs regular INR checks; DOACs need periodic kidney and liver function tests.

Heparin, given by injection, acts faster and is used in hospitals for acute situations.