Heart Home > Side Effects of Warfarin

Blood in the urine, bleeding gums, and unusual bleeding are potentially serious warfarin side effects that should be reported to a healthcare provider right away. Careful monitoring (using a blood test) can help reduce the risk of many of these serious problems. Some of the side effects that are merely bothersome and usually not dangerous include lethargy and weakness, taste changes, and hair loss.

An Introduction to Warfarin Side Effects

Just like any medicine, warfarin sodium (Coumadin®, Jantoven®) can cause side effects, which can be quite dangerous. The risk of most dangerous side effects can be reduced by careful monitoring.
(This article covers many, but not all, of the possible side effects with warfarin. Your healthcare provider can discuss a more complete list of warfarin side effects with you.)

Serious Side Effects Seen With Warfarin

Some warfarin side effects are potentially serious and should be reported immediately to your healthcare provider. These include but are not limited to:
  • Headaches, dizziness, or weakness
  • Unusual bruising (bruises that develop without known cause or grow in size)
  • Blood in the urine
  • Blood in the stool (usually seen as red or black stools)
  • Nosebleeds
  • Bleeding gums
  • Bleeding from cuts that takes a long time to stop
  • Menstrual bleeding or vaginal bleeding that is heavier than normal
  • Coughing up blood
  • Vomiting blood or material that looks like coffee grounds
  • Purple-colored toes
  • Dead, gangrenous tissue
  • Signs of an allergic reaction, such as:
    • An unexplained rash
    • Itching
    • Hives
    • Swelling of the mouth or throat
    • Wheezing
    • Difficulty breathing.
Written by/reviewed by:
Last reviewed by: Kristi Monson, PharmD;
Last updated/reviewed:
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