Anticoagulant medicines - Side effects 

Side effects 

Reporting side effects

The Yellow Card Scheme allows you to report suspected side effects from any type of medicine that you are taking.

It is run by a medicines safety watchdog called the Medicines and Healthcare products Regulatory Agency (MHRA).

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)
  • bleeding gums
  • blood in your vomit or 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

You must also seek immediate medical attention if you:

  • are involved in a major accident
  • experience a significant blow to the head
  • are unable to stop any bleeding

While you are taking anticoagulant medicines, you will be monitored closely 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:

See your GP immediately if you experience jaundice (yellowing of the skin and whites of the eyes) or any persistent side effects while taking warfarin.


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

Page last reviewed: 04/09/2013

Next review due: 04/09/2015


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The 1 comments posted are personal views. Any information they give has not been checked and may not be accurate.

Nicolas1stson said on 11 May 2014

I have been on warfarin since 1995 after I had a stroke at the age of 46. Yes , there are side effects but it keeps you alive! It important to have your blood checked....I have been on thirty mg to zero! I also suffer from very itchy legs and it always starts at night! However I find if after my shower in the morning I use an awful lot of bodylotion .....I go through a bottle week..... if I forget the itching comes back during the night. Itching of the skull is equally awful again I tried different remedies but buy a bottle of neutrgena shampoo and leave for about five minutes and then use another shampoo because the tar smell is awful. Use it about twice a week. It does help. If it really gets bad...and I have had those days....take an antihistamine tablet and after a while you feel the itch ease away. Don't eat too much green and try to avoid stress. Kind regards Hedwig

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