anticoagulant

Blood clotting is like a chain of dominoes: one factor activates the next until a sturdy fibrin mesh forms. Anticoagulants are a drug class that knocks out one or more dominoes in this cascade, so the blood clots more slowly. They are loosely called “blood thinners,” though they do not actually thin the blood — they slow the chemistry of clotting.

These drugs are best at preventing the slow, fibrin-rich clots that form where blood pools or flows sluggishly: in deep leg veins, in the lungs as pulmonary embolism, and inside a fibrillating left atrium where a stroke-causing clot can form. That is a different target from antiplatelet drugs, which guard fast-flowing arteries.

Anticoagulants come in several forms — vitamin-K antagonists like warfarin, direct oral anticoagulants that block a single factor, and injectable heparins for rapid effect. They differ in how quickly they act, whether they need blood monitoring, and how they are reversed in an emergency.

The unavoidable trade-off is bleeding. Because these drugs blunt the body's normal repair of any leak, dosing is chosen carefully and is reduced or held when bleeding risk is high, such as before surgery or in people who fall frequently.