Anticoagulant medicines are used if your blood is clotting too quickly. When this happens, blood clots can form in the wrong places. These clots can break off and block a blood vessel, disrupting the flow of blood around your body.

This can lead to several serious medical conditions, including:

• strokes – when a blood clot restricts the flow of blood to your brain, causing brain cells to die and possibly resulting in permanent brain damage or death
• transient ischaemic attacks (TIAs) – also called "mini-strokes" with symptoms similar to a stroke, but the effects usually only last 24 hours
• heart attacks – when a blood clot blocks part of your heart, starving it of oxygen and causing chest pain and sometimes death
• deep vein thrombosis (DVT) – when a blood clot forms in one of the deep veins in your body, usually your legs, causing pain and swelling
• pulmonary embolism – when a blood clot blocks one of the blood vessels around the lungs, stopping the supply of blood to your lungs

If you are at risk of any of the above conditions, for example if you have had one before, you may be prescribed anticoagulant medicines to reduce your chance of the condition recurring.

Aspirin and clopidogrel are antiplatelet medicines that also reduce the ability of the blood to clot. In some cases, one of the above conditions may be treated with aspirin or clopidogrel instead. The healthcare professionals treating you will explain to you which medication is most suitable.

Increased risk of blood clots

A number of conditions lead to an increased risk of blood clotting in blood vessels. The term used to describe these conditions is thrombophilia.

The majority of cases of thrombophilia that can cause deep vein thrombosis or pulmonary embolism are passed on from your parents. The most common type of inherited thrombophilia is Factor V Leiden.

You may be prescribed anticoagulants to prevent a clot forming if you show a positive result in a screening test. These tests will only be done with your permission and only if a close relative develops a blood clot that cannot be explained.

Certain other conditions can increase the risk of a blood clot, particularly if they reduce the flow of blood in your legs or heart, or affect your immune system.

Reduced flow of blood

A reduced flow of blood in the veins of your leg is usually caused by long periods of not being able to move. This is more likely after certain types of surgery such as a hip or knee replacement. After this type of surgery you may be given a low dose of heparin to help prevent deep vein thrombosis or pulmonary embolism.

A reduced flow of blood in the heart is usually a result of palpitations caused by certain irregular heartbeats (arrhythmias). The most common of these conditions is atrial fibrillation, which can lead to blood clots forming in the heart. People with atrial fibrillation are usually treated with warfarin to reduce the risk of a clot forming.

Immune system

Certain conditions can also affect the immune system, which leads to an increased risk of developing a clot or embolus. Examples of these conditions include:

• antiphospholid syndrome – a disorder of the immune system, where antibodies attack fats and proteins in the blood vessels instead of bacteria and viruses, causing the blood to clot
• paroxysmal nocturnal haemoglobinuria (PNH) – a rare blood disorder that can affect the blood clotting process, leading to an increased risk of blood clots in the abdomen and liver

If you are diagnosed with any of these conditions, you may be prescribed long-term anticoagulants to treat blood clots or to prevent them forming.

After surgery

You may be prescribed an anticoagulant or an antiplatelet agent if you have recently undergone some kinds of surgery, such as aortic valve replacement. Your aortic valve is a valve in your heart that controls the flow of blood out to the rest of your body. This valve can become damaged as you age and in some people it is replaced with a man-made valve.

Blood clots can form on the surface of the new valve, which could disrupt the flow of blood through your heart. Anticoagulant medicines can reduce the risk of this happening by making it harder for your blood to clot.