Anticoagulant medicines

Anticoagulant medicines reduce the ability of the blood to clot (coagulation means clotting). This is necessary if the blood clots too much, as blood clots can block blood vessels and lead to conditions such as a stroke or a heart attack.

The two most common anticoagulant medicines are:

• heparin, and
• warfarin.

Aspirin is an anti-platelet medicine. This is also an anticoagulant, as it reduces the ability of the blood to clot. However, aspirin does this in a slightly different way to the medicines above. For information on aspirin, please see the Health A-Z topic on anti-platelets, aspirin, low dose.

What makes up blood 

Blood is made up of:

• plasma - a liquid containing proteins, nutrients, hormones and waste products, but mainly water (90%),
• red blood cells - to transport oxygen around the body and remove carbon dioxide and other waste products,
• white blood cells - to fight infection, and
• platelets - to help the blood to clot.

When there is a wound, either on the skin or inside the body, blood can leak out of the body, or into the internal organs. To prevent this, the blood forms clots that create a seal over the wound.

How the blood clots

When the blood needs to clot, a series of complex chemical processes take place in the blood. These chemical processes change the surface of the platelets so that they become sticky (known as ‘activated’). The platelets then stick to the site of the bleeding and start to form a clot.

The activated platelets also cause a number of other chemical reactions that activate clotting factors in the blood. Clotting factors are chemicals or proteins that are produced by the liver. There are 13 clotting factors that can be found in the plasma in the blood.

One of these clotting factors is a protein called prothrombin. During the clotting process, prothrombin turns into an enzyme called thrombin. An enzyme is a protein that speeds up and controls chemical reactions in your body. In this case, the thrombin acts upon a protein in the blood called fibrinogen, and turns it into a different protein, called fibrin.

Fibrin is like sticky strands of string. These strands stick to the site of the wound and form a web. Red blood cells stick to the web and start to form a clot. As the fibrin and the platelet clots bind together, a solid clot is created that stops further bleeding.   

Why is anticoagulant medicine necessary?

The process of forming clots is complicated and relies on a lot of processes happening correctly. If one or more parts of the process fail to work, then the blood can clot too much, or not enough.

If the blood does not clot enough, there is a risk of excessive bleeding (haemorrhaging). If it clots too much, then blood clots can form where they are not needed, and could block blood vessels. Anticoagulant medicines can reduce the ability of the blood to clot so that unnecessary blood clots are not formed.

Who takes anticoagulant medicines?

Anticoagulant medicines are most commonly prescribed for people who are at risk of developing conditions caused by blood clots, such as stokes. This includes:

• people over 65 - as this age group is more at risk of strokes,
• men - as men are more likely to have heart problems than women,
• people of Afro-caribbean origin - as they have a genetic predisposition (a natural tendency) to develop heart disease and high blood pressure (hypertension), which can cause strokes, and
• people who are overweight or have a diet high in fat - as they are more likely to have other health problems, such as high blood pressure, which can lead to strokes or heart problems.