An anticoagulant, commonly known as a blood thinner, is a chemical substance that prevents or reduces the coagulation of blood, prolonging the clotting time. Some occur naturally in blood-eating animals, such as leeches and mosquitoes, which help keep the bite area unclotted long enough for the animal to obtain blood.

As a class of medications, anticoagulants are used in therapy for thrombotic disorders. Oral anticoagulants (OACs) are taken by many people in pill or tablet form, and various intravenous anticoagulant dosage forms are used in hospitals. Some anticoagulants are used in medical equipment, such as sample tubes, blood transfusion bags, heart–lung machines, and dialysis equipment. One of the first anticoagulants, warfarin, was initially approved as a rodenticide.

Anticoagulants are closely related to antiplatelet drugs and thrombolytic drugs by manipulating the various pathways of blood coagulation. Specifically, antiplatelet drugs inhibit platelet aggregation (clumping together), whereas anticoagulants inhibit specific pathways of the coagulation cascade, which happens after the initial platelet aggregation but before the formation of fibrin and stable aggregated platelet products.

Common anticoagulants include warfarin and heparin.

The use of anticoagulants is a decision based on the risks and benefits of anticoagulation. The biggest risk of anticoagulation therapy is the increased risk of bleeding. In otherwise healthy people, the increased risk of bleeding is minimal, but those who have had recent surgery, cerebral aneurysms, and other conditions may have too great a risk of bleeding. Generally, the benefit of anticoagulation is preventing or reducing the progression of a thromboembolic disease. Some indications for anticoagulant therapy that are known to have benefit from therapy include:

In these cases, anticoagulation therapy prevents the formation or growth of dangerous clots.

The decision to begin therapeutic anticoagulation often involves the use of multiple bleeding risk predictable outcome tools as non-invasive pre-test stratifications due to the potential for bleeding while on blood thinning agents. Among these tools are HAS-BLED, ATRIA, HEMORR2HAGES, and CHA2DS2-VASc. The risk of bleeding using the risk assessment tools above must then be weighed against thrombotic risk to formally determine the patient's overall benefit in starting anticoagulation therapy.

There is no evidence to indicate that adding anticoagulant therapy to standard treatment has a benefit for people with cerebral small vessel disease but not dementia, and there is an increased risk of a person with this disease experiencing a bleed with this approach.