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Anticoagulant medicines - Side effects

NHS Choices Medical Reference

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A side effect of all anticoagulants is the risk of excessive bleeding (haemorrhages). This is because these medicines increase the time it takes blood clots to form. If they take too long, you can experience excessive bleeding.

Other side effects

As well as excessive bleeding, there are other symptoms to look out for. These are more common with warfarin. If you notice any of the following symptoms when taking anticoagulants, seek medical attention immediately:

  • passing blood in your urine, or faeces (stools)
  • passing black faeces
  • severe bruising
  • prolonged nosebleeds (lasting longer than 10 minutes)
  • blood in your vomit
  • coughing up blood
  • unusual headaches
  • sudden severe back pain
  • difficulty breathing or chest pain
  • (in women) heavy or increased bleeding during your period or any other bleeding from your vagina

While you are taking anticoagulant medicines, you will be closely monitored to check that you are on the correct dose and not at risk of excessive bleeding (haemorrhages). The most common test for this is the international normalisation ratio (INR).


Additional side effects caused by warfarin include:

  • rashes
  • diarrhoea
  • nausea (feeling sick)
  • vomiting


Although heparin occurs naturally within the body, extra amounts of it can cause side effects, including:

  • hair loss (alopecia)
  • thrombocytopenia - an abnormal drop in the number of platelets in your blood, which can cause bleeding into vital areas
Medical Review: November 23, 2011
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