Researchers from the Cheyenne Veterans Affairs Medical Center, in Wyoming report that when taken together, 1 patient experienced bleeding from the blood “thinner” warfarin (Coumadin).

First, the details.

  • A 56-year-old man had been treated with warfarin for 11 years after heart valve replacement.
  • He came to the hospital with upper gastrointestinal bleeding.
  • His INR value was very high at 10.41, and his hemoglobin level was low at 6.6 g/dL.
    • The INR test measures the ability of blood clot: normal is 0.8 to 1.2.
    • A normal hemoglobin count is 14-17 g/dL for men.
  • He was treated to control his blood loss and left the hospital after 7 days.
  • 15 days later he returned with a constant nosebleed and increased bruising.
  • His INR value was even higher at 11.55.
  • Again, he was treated.
  • It was learned that the patient smoked marijuana more frequently throughout the period of these 2 hospitalizations due to depression.
  • He was counseled about the potential interaction of warfarin and marijuana.
  • He stopped smoking pot, and during the next 9 months his INR values ranged from 1.08 to 4.40 with no significant bleeding complications.

The bottom line?
This is the first reported case of a warfarin-marijuana interaction. More clinical reports would be useful to confirm this interaction.

A PubMed search since this research was published revealed nothing new on this topic.

6/17/09 19:08 JR; 2/3/11 21:52 JR

Hi, I’m JR

John Russo, Jr., PharmD, is president of The MedCom Resource, Inc. Previously, he was senior vice president of medical communications at, a complementary and alternative medicine website.