Side Effects Of Anticoagulant Medications

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Anticoagulants, often known as blood thinners, are an important line of defence against hazardous blood clots, which can cause strokes and heart attacks. This article explains all you need to know about these lifesaving drugs. We'll discuss what anticoagulants are, why they're used, and popular varieties such as Warfarin. 

We'll also go into the key point: side effects, which can range from modest bruises to severe bleeding and adverse reactions. In addition, we'll discuss how to manage these side effects and what to look out for in terms of medication interactions. Finally, we'll provide some helpful recommendations for anybody using anticoagulants, such as what to inform your doctor before surgery or if you're pregnant, to ensure your safety while taking these medicines.

What are anticoagulant medications?

Anticoagulant medications prevent blood clots and are also known as blood thinners. However, this name can be misleading as anticoagulants do not cause your blood to become thin. Blood clots are made of blood cells known as platelets, and a protein called fibrin.1 You will be prescribed anticoagulant medications if you have a high risk of developing blood clots.

Purpose of anticoagulants in medical treatment

Anticoagulant medications are commonly prescribed if you have one of the following conditions, or to prevent them from occurring:1

The above conditions heighten your risk of developing a harmful blood clot in your heart, and if this blood clot travels through your bloodstream to the brain and blocks a blood vessel which carries blood to your brain, this can cause a stroke.1

Common anticoagulant medications

The most commonly prescribed blood thinner is warfarin.

What is warfarin?

  • Warfarin is an oral anticoagulant that is prescribed to treat blood clots.
  • Warfarin treats blood clots by reducing the amount of active vitamin K in your body.
  • Warfarin is typically taken once a day, typically in the afternoon or evening.
  • If you are prescribed warfarin please be advised that there is an increased risk of bleeding and bruising easily
  • Adverse side effects include haemorrhage (bleeding profusely)2

Which other anticoagulants may you be prescribed?

Warfarin has been prescribed as the main anticoagulant medication for over six decades. However, warfarin is starting to be replaced by direct-acting oral anticoagulants, such as the following:3

Please note that as well as direct-acting oral anticoagulants, you may also be prescribed an anticoagulant medication known as heparin, which is given as an injection rather than orally.

Why are direct-acting oral anticoagulants prescribed over warfarin?

Direct-acting oral anticoagulants are preferred over warfarin by doctors due to the following reasons:4

  • They have a much more rapid onset, meaning that the onset of their therapeutic effect (preventing blood clots) on your body is much quicker.
  • They have fewer interactions with other drugs, which can result in unwanted effects and the anticoagulant medication being less effective at preventing blood clots.
  • They do not require frequent blood monitoring like warfarin, which requires frequent blood tests.
  • They have fewer food interactions.

Mechanism of action: how do anticoagulant medications work to slow down the blood clotting process?

What is the mechanism of action of warfarin?

Warfarin binds to an enzyme in your body known as vitamin K epoxide reductase complex 1, which is needed in the production of vitamin K in your body. If warfarin binds to this enzyme there is less production of active vitamin K in your body. Why is this important? Well, active vitamin K is needed in the blood clotting process to make active clotting factors. Less active clotting factors means that there is less blood clotting.2

What are the side effects of anticoagulant medications?


One of the major side effects of taking anticoagulant medications is bleeding, with the risk being greater when you are prescribed warfarin over any other anticoagulant medication.  Bleeding can occur if an anticoagulant is inappropriately administered or if the dose of the anticoagulant medication is too high. Patient education is important to understand and minimise the risk of bleeding when on anticoagulant medication. 

In addition, before you start a course of warfarin treatment you should be assessed by a doctor to ensure the medication and dosage are appropriate for you, and you should be monitored throughout your treatment.5 

Gastrointestinal bleeding

All anticoagulants, both warfarin and direct-acting oral anticoagulants, carry a risk of causing gastrointestinal bleeding if there is any pre-existing damage or abnormal change to the tissue of the gastrointestinal tract, which can be caused by disease or physical trauma. However, there are no definite conclusions about which anticoagulant has the lowest risk of causing gastrointestinal bleeding.6

Intracranial haemorrhage

Another side effect of taking anticoagulant medications is intracranial haemorrhage. Intracranial haemorrhage is when there is a brain bleed.7 The use of anticoagulant medications is associated with a higher risk of causing a brain bleed.8 However, direct-acting oral anticoagulants have been associated with a lower risk of causing an intracranial haemorrhage compared to medications such as warfarin.8


Bruising occurs when the smallest blood vessels under your skin, known as capillaries, rupture, causing blood to leak out of them. The blood leaking out of the capillaries causes discolouration, which is what is known as a bruise. When you are on anticoagulant medications this can cause you to bruise more easily, or come out in larger, more discoloured bruises. This is due to anticoagulant medications slowing down the blood clotting process, and if your blood cannot clot this causes more blood to leak out of the ruptured capillaries.


Due to anticoagulants having the side effect of severe bleeding, this can potentially result in anaemia.9 Anaemia is when the amount of red blood cells in your bloodstream rapidly declines. Your red blood cells contain a protein known as haemoglobin, which carries oxygen to your body’s tissues. Having fewer red blood cells means that less oxygen is carried to your body’s tissues, which is needed for your cells and tissues to survive. This can result in the following symptoms:

  • Fatigue
  • Shortness of breath
  • Dizziness

Adverse reactions

Heparin is a type of anticoagulant which stops the blood clotting process. However, heparin can unfortunately lead to an adverse reaction known as heparin-induced thrombocytopenia. This is a response from your immune system which causes antibodies to activate cells in your blood known as platelets, which are responsible for clotting. This in turn leads to an increased risk of blood clotting.10

Other side effects

Other notable side effects include the following:

  • Having heavy periods if you are assigned female at birth
  • Bleeding gums
  • Vomiting blood
  • Prolonged nosebleeds
  • Passing blood in your urine or faeces

Drug interactions

Drug interactions can take place when you take an anticoagulant medication combined with another drug, and this can unfortunately lead to an increased risk of bleeding. The following types of medications can interact with anticoagulant medications to elicit this effect:11

Drug interactions and warfarin

It is also important to note that antibiotics, antifungals and antivirals can interfere with the mechanism of action of certain anticoagulant medications, including warfarin.13

Antibiotics interfere with your gut flora’s production of vitamin K, and vitamin K has the ability to reverse the anticoagulant effects of warfarin.13

In addition, statins can also interfere with the mechanism of action of warfarin, which includes the following:13

The following antiplatelet drugs can also interfere with warfarin and cause excessive bleeding:13

The antidepressant drug, fluoxetine, can also interfere with the action of warfarin in your body, and an alternative antidepressant should be prescribed by your doctor if you are on warfarin.13

Please note that grapefruit juice and cranberry juice can both interfere with the mechanism of action of warfarin, and grapefruit juice consumption should not exceed more than 200 ml a day when you are taking warfarin, and your cranberry juice consumption should not exceed more than 24 ounces a day.13

Please note that this list of drug-to-drug interactions is not exhaustive, and you should speak to your pharmacist or doctor about which other medications are appropriate to take in conjunction with warfarin or any other anticoagulant medication.

Monitoring and management of side effects

Regular follow-up

If you are on warfarin you will need to have routine international normalised ratio (INR) blood tests, which will assess how long it takes for your blood to clot. Direct-acting oral anticoagulants do not require this type of monitoring, but you will still have to regularly follow up with your doctor. This will allow your treatment to be reviewed and you can be assessed to see if you are experiencing any serious side effects, such as bleeding.

Adjustment of dosage

If you are on warfarin and have an INR test, the doctor will see if the INR value is within the target to ensure that the warfarin is having the desired effect on the body, and if it is not within range the doctor may change your dosage. This ensures that the warfarin dose is safe for you, to avoid any unwanted side effects, and that it is having the desired effect.13

Patient considerations and pre-existing conditions affecting anticoagulant use

Surgery and tooth removal

If you are having surgery or a tooth removed it is important to let your dentist or doctor know that you are taking anticoagulants as they affect the ability of your blood to clot. You may be advised to stop taking your anticoagulant medication before surgery, or you may need a blood test to see if your blood clots at the correct speed to have the procedure. Pregnancy and breastfeeding

You should not normally take warfarin if you are pregnant, and in your first or third trimester, as this can affect your unborn child and result in birth defects, or possibly excessive bleeding from your placenta or unborn child. In the second trimester of your pregnancy, you may be advised to take warfarin. In addition, the newer anticoagulant medications, such as direct-acting oral anticoagulant medications are not recommended during pregnancy.

If you are breastfeeding the advice does change, and you may be able to take warfarin or heparin, but you must consult with a GP first. Direct-acting oral anticoagulants are not recommended when you are breastfeeding as it is unsure whether or not they cause harm to the baby.


  • Anticoagulant medications slow down the process of blood clotting.
  • You may be prescribed anticoagulant medications if you have atrial fibrillation, or have had artificial heart valves, as examples.
  • Side effects of taking anticoagulants include excessive bleeding, bruising, anaemia and possible adverse reactions to heparin.
  • Anticoagulant medications can interact with other medications, so please seek advice from your pharmacist or doctor before starting another medication.
  • Please make sure you are following up with your healthcare provider whilst on anticoagulant medication, this ensures risks are managed and you are not experiencing any adverse side effects.
  • Please ensure you take the appropriate advice regarding your anticoagulant medication if you are pregnant, breastfeeding, going for surgery, or having a tooth removed.


  1. Medicines for my heart. British Heart Foundation [Internet]. [cited 2024 Apr 29]. Available from:
  2. Patel S, Singh R, Preuss CV, Patel N. Warfarin. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 [cited 2024 Jan 10]. Available from:
  3. Vinogradova Y, Coupland C, Hill T, Hippisley-Cox J. Risks and benefits of direct oral anticoagulants versus warfarin in a real world setting: cohort study in primary care. BMJ [Internet]. 2018 [cited 2024 Jan 10]; 362:k2505. Available from:
  4. Wei S, Sawhney A, Khandait H, Meda A, Gupta V, Jain R. An update on applications and limitations of direct oral anticoagulants. The Egyptian Journal of Internal Medicine [Internet]. 2023 [cited 2024 Jan 10]; 35(1):26. Available from:
  5. Amaraneni A, Chippa V, Rettew AC. Anticoagulation Safety. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 [cited 2024 Jan 10]. Available from:
  6. Martin A-C, Benamouzig R, Gouin-Thibault I, Schmidt J. Management of Gastrointestinal Bleeding and Resumption of Oral Anticoagulant Therapy in Patients with Atrial Fibrillation: A Multidisciplinary Discussion. Am J Cardiovasc Drugs [Internet]. 2023 [cited 2024 Jan 11]; 23(4):407–18. Available from:
  7. Caceres JA, Goldstein JN. Intracranial Hemorrhage. Emerg Med Clin North Am [Internet]. 2012 [cited 2024 Jan 11]; 30(3):771–94. Available from:
  8. Wu T, Lv C, Wu L, Chen W, Lv M, Jiang S, et al. Risk of intracranial hemorrhage with direct oral anticoagulants: a systematic review and meta-analysis of randomized controlled trials. J Neurol [Internet]. 2022 [cited 2024 Jan 11]; 269(2):664–75. Available from:
  9. Killeen RB, Tambe A. Acute Anemia. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 [cited 2024 Jan 12]. Available from:
  10. Ahmed I, Majeed A, Powell R. Heparin induced thrombocytopenia: diagnosis and management update. Postgrad Med J [Internet]. 2007 [cited 2024 Jan 12]; 83(983):575–82. Available from:
  11. Vazquez SR. Drug-drug interactions in an era of multiple anticoagulants: a focus on clinically relevant drug interactions. Hematology Am Soc Hematol Educ Program [Internet]. 2018 [cited 2024 Jan 12]; 2018(1):339–47. Available from:
  12. Mar PL, Gopinathannair R, Gengler BE, Chung MK, Perez A, Dukes J, et al. Drug Interactions Affecting Oral Anticoagulant Use. Circ: Arrhythmia and Electrophysiology [Internet]. 2022 [cited 2024 Jan 12]; 15(6). Available from:
  13. Kuruvilla M, Gurk-Turner C. A review of warfarin dosing and monitoring. Proc (Bayl Univ Med Cent) [Internet]. 2001 [cited 2024 Jan 12]; 14(3):305–6. Available from:

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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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Alisha Solanki

BSc Biomedicine, Lancaster University

Current biomedical science student with a keen interest in medical communications. I have a passion for producing scientifically correct articles in plain language, and communicating advances in the biomedical field to the public.

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