Side effects 

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

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).

Page last reviewed: 04/09/2013

Next review due: 04/09/2015