What Is Anticoagulant Therapy

  • Catherine Crocker Medical Doctor - BMBS, University of Southampton, United Kingdom
  • Yuna Chow BSc (Hons), Medicine, University of St Andrews


Anticoagulant therapies (also known simply as anticoagulants) are medications designed to prevent blood clots from forming, or to dissolve clots that are already present in the circulatory system.1 You may have heard the term ‘blood thinners’, which is an informal name given to both anticoagulants and medications such as aspirin that stop platelets clumping together, thus preventing blood clots.2

Blood clotting (also known as coagulation) is essential in the body. For example, if you cut yourself it it is important your blood clots to prevent uncontrolled bleeding.2 However, large clots can also cause blockages in blood vessels, reducing and potentially stopping oxygen getting to organs, causing damage known as ischaemia.3

Conditions that can be caused by a blood clot:1,3,4

  • Stroke - can be caused by a blocked blood vessel, causing irreversible symptoms
  • Transient Ischaemic Attack - sometimes called a ‘mini stroke’, a blood vessel in the brain is momentarily blocked but then clears with no lasting symptoms
  • Heart Attack - a blood clot blocks a branch of the coronary arteries, causing damage to the heart’s muscle
  • Pulmonary Embolism - a blood clot blocks a pulmonary artery, preventing oxygenated blood reaching the lung tissue5

Types of anticoagulants

There are several different anticoagulants with different uses, warfarin being the most common:1,3

  • Warfarin 
  • Direct Oral Anticoagulants, also known as DOACs - dabigatran, rivaroxaban and apixaban are examples of these 
  • Heparin - an injectable anticoagulant, which comes in two forms; unfractionated heparin and low molecular weight heparin (LMWH) 
  • Fondaparinux - a heparin-like medication 

How anticoagulants work 

Blood clots through a complex system of vitamins and enzymes that activate when a blood vessel is damaged - known collectively as clotting factors.6 Clotting factors rely on a specific chain of events (known as the coagulation cascade) to be successful to form blood clots.   

Anticoagulants disrupt this sequence in different ways, depending on the medication:1,6,7 

  • Warfarin - blocks the effect of vitamin K on the coagulation cascade, which is essential for the activation of clotting factors 
  • Direct Oral Anticoagulants - act on different enzymes in the coagulation cascade, depending on the medication. These all have the same effect, and stops blood clots being formed
  • Heparin - Unfractionated heparin starts and finishes working quickly once given, whilst LMWH is more slow-acting – both bind to proteins and enzymes
  • Fondaparinux works in the same manner as low molecular weight heparin, but is less strong and therefore more effective in preventing blood clot formation than treating formed clots

Indications for anticoagulation therapy

Anticoagulants are prescribed where a person already has a blood clot, or has a condition where developing blood clots is more likely.

The following conditions may be treated with anticoagulants:8,9

  • Atrial Fibrillation - sometimes described as an ‘irregular heartbeat’, this can cause blood clots to form in the heart
  • Deep Vein Thrombosis (DVT) - a blood clot already present in one of the leg veins; these can sometimes travel and lodge in the lung vessels and are a common cause of pulmonary embolism
  • Pulmonary Embolism (PE) - although not always caused by a blood clot, ones that are will need anticoagulants
  • Stroke - anticoagulants are sometimes given after emergency treatment to reduce the risk of having another blood clot
  • Blood clotting disorders - especially thrombophilias, which are conditions where blood has an increased tendency to clot and causes a number of symptoms that require treatment with anticoagulants
  • Antiphospholipid Syndrome - an autoimmune condition where the body attacks proteins in the blood

Anticoagulants are also used in surgery to prevent blood clots being caused by the operation or the recovery period:9

  • Heart valve surgery or replacement - blood clots are more prone to develop when a heart valve has been operated on
  • Any operation where movement is going to be limited in the recovery period, as blood clots are more common when immobile. This is particularly common afteri hip and knee surgery 

Monitoring and dosage adjustments

Anticoagulants are effective in treating and preventing blood clots, but it is important their blood levels are monitored to ensure they are neither too low, risking clots, or too high, risking side effects such as bleeding.

  • Warfarin - the International Normalised Ratio (INR) is a blood test used to monitor blood warfarin levels. The INR needs checking regularly and your warfarin dose might need to be changed a number of times before to find the right dose for you. Owing to the relationship between warfarin and vitamin K, patients may also need to monitor their intake of vitamin K rich foods, like leafy green vegetables, liver and chickpeas.10,11 Warfarin must be taken as prescribed and missing doses should be avoided, to ensure the INR is as stable as possible.12 
  • DOACs work much faster than warfarin, so tend to remain at a stable level in the blood. This means they need very few, or even no blood tests. DOACs may need more than one dose to be taken each day, to counter their short-acting nature.Intake should be strict and regular to ensure there is always full cover.10,12
  • Low Molecular Weight Heparin is sometimes used in the transition to oral anticoagulants, until a stable dose of warfarin has been established. This is injected daily, usually as an outpatient but this may also happen in hospital whilst transferring to warfarin.10

Potential risks and side effects

As anticoagulants reduce the ability for blood to clot, there are some specific risks related to their use.

Risk of bleeding 

Anyone taking anticoagulants is at an increased risk of bleeding, and so must take special care in the following areas:12

  • Surgery - before undergoing any surgical or investigative procedure, it is important to inform the healthcare provider of any anticoagulant use as dose changes may need to be made prior to the procedure 
  • Injury - owing to the increased risk of bleeding even small injuries can become more serious, so taking precautions to avoid receiving cuts and grazes is very important. You may also be advised to avoid participating in contact sports
  • Food and drink interactions warfarin - grapefruit juice, cranberry juice and pomegranate juice increase the effect of warfarin, making bleeding more likely. Alcohol intake should also be limited to no more than two alcoholic drinks per day, and binge drinking is to be entirely avoided. 

Interaction with other medications 

Anticoagulants can interact with a number of different medications so it is essential to let your healthcare professional know you are taking them. Some of the medications anticoagulants can interact with are as follows:11

If unsure about any possible interaction it is important to speak to a medical professional before taking a new medication.

Signs of bleeding

It is important to recognise signs that anticoagulants may be either causing bleeding or not fully preventing blood clots. Seeking advice from a medical professional is essential with any of these symptoms:1

  • Bruising easily, or having bruises you cannot explain
  • Unable to stop bleeding from a minor injury, such as a cut or graze
  • Feeling dizzy, weak or tired - as this may be a sign of internal bleeding

It is also important to seek emergency assistance if you experience any of the following. Even without any visible bleeding it is important to be checked for any signs of internal bleeding:1

  • Vomiting blood, or coughing up a black, granular substance sometimes known as ‘coffee ground’ vomit
  • Passing blood or black stools when you use the toilet
  • Blood visible when passing urine 
  • Severe stomach pain
  • Any head injury - either hitting your head or receiving a blow to the head
  • Any injury where you have been crushed
  • Any car accident

Signs of clotting

Signs that you may still be experiencing symptoms of a blood clot, whilst taking anticoagulation, will depend on the cause and location of the blood clot. However, you should seek emergency assistance if you experience any of the following symptoms whilst taking anticoagulants as it may be indicative of further issues with a pre-existing blood clot or a new clot:13

  • A combination of swelling, tenderness, redness and pain in a limb - may be indicative of a DVT 
  • Sudden shortness of breath, coughing up blood - may be indicative of a PE 
  • Chest pain, feeling faint or dizzy, fast or irregular heart rate - may be indicative of a heart attack or a PE

Pregnancy and anticoagulation

Warfarin is generally not given during pregnancy, especially during the first trimester and third trimester, owing to its potential to cause birth defects and miscarriage.11 Additionally DOACs are not recommended in pregnancy, and so low molecular weight heparin is the anticoagulant of choice when expecting.11

Elderly patients

In elderly patients with atrial fibrillation, there is a need to balance the increased risk of having an illness caused by a blood clot and the risk of increased bleeding following anticoagulation, especially with an increased risk of falling over. This is assessed using the CHA2DS2VASc and HAS-BLED risk scoring systems to determine risk of having a blood clot and risk of bleeding respectively. This determines whether anticoagulation is required, and whether additional medication and lifestyle interventions are required to reduce the likelihood of falling.14

Future trends and developments

Although DOACs are more stable and easier to manage than warfarin, there are still some significant bleeding risks associated with their use. New DOACs that work on a different part of the ‘coagulation cascade’ are being researched to ascertain if they have less bleeding risks, but they are still in a trial phase.15


What does anticoagulant therapy do?

Anticoagulants are used for conditions where formation of blood clots needs to be prevented, or to treat blood clots that are already present. This is achieved by blocking the effects of enzymes and vitamins on the ‘coagulation cascade’, which normally causes blood to clot.

Does anticoagulant mean blood thinner?

‘Blood thinner’ is an informal way of describing any medication that prevents or reduces blood clotting. Therefore anticoagulants could be described as blood thinners, but other medications that work in different ways, like aspirin, could be described in the same way.

What conditions are treated with anticoagulants?

Anticoagulants are used to treat many conditions, such as atrial fibrillation, deep vein thrombosis, pulmonary embolism and long-term treatment after a stroke caused by a blood clot. They are also used to prevent blood clots forming in heart surgery and joint replacement procedures. 

Is aspirin considered anticoagulant therapy?

Although aspirin has the same effect as anticoagulant therapy, it is an ‘antiplatelet’ medication. Aspirin stops platelets in the blood clumping together and causing clots, rather than stopping the chemical process of coagulation.


Anticoagulants are a group of medications designed to prevent or treat blood clots in the body’s circulation system. There are three main types, the vitamin K blocker warfarin, DOACs such as apixaban and a group of injectable medicines called heparins. Conditions such as atrial fibrillation, pulmonary embolism and deep vein thrombosis are treated with anticoagulants, and they are also given in operations associated with a higher risk of blood clots. 

As anticoagulants target blood clotting, they can cause bleeding and will need to be taken regularly. Warfarin is also monitored with regular INR blood tests. It is also important to be aware that other medications and even food or drink may interact with anticoagulant therapy.  

The main side effect of anticoagulant therapy is bleeding and so extra care must be taken to avoid injuries. Any signs of mild to moderate uncontrolled bleeding or unexplained bruising need to be discussed with a healthcare professional. Immediate medical attention is required with all head injuries, crush injuries and car accidents, as well as any severe uncontrolled bleeding. 


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  • Hoffman M, MD. WebMD. [cited 2023 Nov 27]. Blood clots. Available from: https://www.webmd.com/dvt/blood-clots
  • nhs.uk [Internet]. 2018 [cited 2023 Nov 27]. Anticoagulant medicines. Available from: https://www.nhs.uk/conditions/anticoagulants/
  • Mayo Clinic [Internet]. [cited 2023 Nov 27]. Transient ischemic attack (Tia) - Symptoms and causes. Available from: https://www.mayoclinic.org/diseases-conditions/transient-ischemic-attack/symptoms-causes/syc-20355679
  • Pulmonary embolism [Internet]. 2021 [cited 2023 Nov 27]. Available from: https://www.hopkinsmedicine.org/health/conditions-and-diseases/pulmonary-embolism
  • Palta S, Saroa R, Palta A. Overview of the coagulation system. Indian J Anaesth [Internet]. 2014 [cited 2023 Nov 28];58(5):515–23. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4260295/
  • Hsu E, Moosavi L. Biochemistry, antithrombin iii. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 [cited 2023 Nov 28]. Available from: http://www.ncbi.nlm.nih.gov/books/NBK545295/
  • nhs.uk [Internet]. 2018 [cited 2023 Nov 29]. Anticoagulant medicines - Uses. Available from: https://www.nhs.uk/conditions/anticoagulants/uses/
  • nhs.uk [Internet]. 2017 [cited 2023 Nov 29]. Stroke - treatment. Available from: https://www.nhs.uk/conditions/stroke/treatment/
  • Frysh P. WebMD. [cited 2023 Nov 29]. What are the types of anticoagulants? Available from: https://www.webmd.com/dvt/anticoagulant-types
  • nhs.uk [Internet]. 2018 [cited 2023 Nov 30]. Anticoagulant medicines - Dosage. Available from: https://www.nhs.uk/conditions/anticoagulants/dosage/
  • nhs.uk [Internet]. 2018 [cited 2023 Nov 30]. Anticoagulant medicines - Considerations. Available from: https://www.nhs.uk/conditions/anticoagulants/considerations/
  • CDC. Centers for Disease Control and Prevention. 2020 [cited 2023 Nov 30]. What is venous thromboembolism? | cdc. Available from: https://www.cdc.gov/ncbddd/dvt/facts.html
  • Atrial fibrillation | royal brompton & harefield hospitals [Internet]. [cited 2023 Nov 30]. Available from: https://www.rbht.nhs.uk/our-services/atrial-fibrillation
  • Bentounes NK, Melicine S, Martin AC, Smadja DM, Gendron N. Development of new anticoagulant in 2023: Prime time for anti-factor XI and XIa inhibitors. JMV-Journal de Médecine Vasculaire [Internet]. 2023 Apr 1 [cited 2023 Nov 30];48(2):69–80. Available from: https://www.sciencedirect.com/science/article/pii/S2542451323000500
This content is purely informational and isn’t medical guidance. It shouldn’t replace professional medical counsel. Always consult your physician regarding treatment risks and benefits. See our editorial standards for more details.

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Catherine Crocker

Medical Doctor - BMBS, University of Southampton, United Kingdom

Catherine is a medical doctor turned carer for her husband, who has been living with Motor Neurone Disease for a number of years. She has refocused her interest in medicine and mental wellness towards medical writing, hoping to help the public understand their own health and make more empowered decisions. Catherine is a keen knitter and keeper of two boisterous cats.

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