Does fish oil cause blood thinning?

Omega-3 fatty acids from fish oil have the capacity to "thin the blood." In reality, omega-3s exert a mild platelet-blocking effect (platelet activation and "clumping" are part of clot formation), while also inhibiting arachidonic acid formation and thromboxane.

But can fish oil cause excessive bleeding?

This question comes up frequently in the office, particularly when my colleagues see the doses of fish oil we use for cardiovascular protection. "Why so much fish oil? That's too much blood thinning!"

The most recent addition to the conversation comes from a Philadelphia experience reported in the American Journal of Cardiology:

Comparison of bleeding complications with omega-3 fatty acids + aspirin + clopidogrel--versus--aspirin + clopidogrel in patients with cardiovascular disease.(Watson et al; Am J Cardiol 2009 Oct 15;104(8):1052-4).

All 364 subjects in the study took aspirin and Plavix (a platelet-inhibiting drug), mostly for coronary disease. Mean dose aspirin = 161 mg/day; mean dose Plavix = 75 mg/day. 182 of the subjects were also taking fish oil, mean dose 3000 mg with unspecified omega-3 content.

During nearly 3 years of observation, there was no excess of bleeding events in the group taking fish oil. (In fact, the group not taking fish oil had more bleeding events, though the difference fell short of achieving statistical significance.) Thus, 3000 mg per day of fish oil appeared to exert no observable increase in risk for bleeding. This is consistent with several other studies, including that including Coumadin (warfarin), with no increased bleeding risk when fish oil is added.

Rather than causing blood thinning, I prefer to think that omega-3 fatty acids from fish oil restore protection from abnormal clotting. Taking omega-3 fatty acids from fish oil simply restores a normal level of omega-3 fatty acids in the blood sufficient to strike a healthy balance between blood "thinning" and healthy blood clotting.

Comments (20) -

  • Marc

    10/26/2009 9:46:32 PM |

    Long time reader, first comment.
    Thank you for so freely sharing all the information.


  • Daniel

    10/26/2009 11:02:46 PM |

    Thank you for this!  I have had this question for a long time given the number of things I take that "thin the blood."

  • Kevin

    10/26/2009 11:44:45 PM |

    As a veterinarian I've dispensed fish oil capsules for several years.  Some owners give so many that the dogs smell 'fishy' when seen for routine care.  The owner doesn't smell it since they're with the dog a lot.  The coats are gorgeous, something that doesn't often happen in Wyoming at 7000ft altitude.

  • Dr. William Davis

    10/26/2009 11:47:45 PM |

    Hi, Kevin--

    My two Boston terriers jump for their fish oil capsules, two every day!

    I'm glad to hear from a veterinarian that the coat sheen is indeed from the fish oil.

  • Rich

    10/27/2009 1:27:09 AM |

    Due to an afib episode a couple of years ago, I was taking 20 mg of warfarin per day, plus around 5000 mg of EPA+DHA, and never had bleeding issues.  

    My INR was always a stable 2.0.

    As I've not had an afib reoccurrence, I've replaced the 20mg coumadin with 325mg aspirin daily, and still take around 5000 mg EPA+DHA.  No bleeding issues with that combo either.

  • Catherine

    10/27/2009 3:55:32 AM |

    Glad this topic came up.
    Over the last 5 years, I've had to periodically eliminate my fish oil intake as I would start to bruise badly. My internist said she has seen this occasionally with fish oil and called it "capillary fragility." I bruise easily anyway, but it would really get bad with fish oil. So there must be some quality in fish oil that influences this.

    Then about 6 months ago I started a strong supplement change to help with my low bone density--already taking magnesium and calcium but added:
    Boron, K2, silica,pomegrantate juice, and BIG increase in vitamin D.
    I also increased omegas to 3,000 a day which I was not able to tolerate before.

    It has been over 4 months since I have had ANY bruise---which is just unheard of for me. I usually have 3-4 different bruises on arms/legs. So something in these supplements  strengthened my capillaries I guess, and I can now take high fish oil doses!
    Anyone else had a bruising problem with fish oil?

  • Dr. William Davis

    10/27/2009 11:04:59 AM |

    Hi, Catherine--

    Fascinating observation!

    I'll bet it has something to do with the vitamin D, more than anything else. Vitamin D seems to strengthen structural tissues in bones, muscle, heart valves, and perhaps capillaries and other small blood vessels.

  • trix

    10/27/2009 11:59:37 AM |

    Several years ago I bruised easily for a while and attributed it to taking garlic supplements daily.  I started taking Vit C and the bruising stopped.  I don't think it had to do with fish oil (in my case); I don't think I was taking fish oil at the time.

  • Daniel

    10/27/2009 9:37:33 PM |

    I too achieve rapid blood thinning when taking 2400mg of EPA/DHA per day. That's only 4 pharmaceutical grade capsules. Even after my vitamin d levels were normalized I still got bruising.

    I now take Vitamin K2 (MK-7 natto extract) twice a week and it's allowed me to bump my EPA/DHA up to 3600mg with no ill effects or bruising.

    It was either supplement or eat a lot of aged cheese, they both seemed to do the trick in my particular case.

  • Healthy Oil Guy

    10/27/2009 9:53:51 PM |

    Thank you for sharing this study with us.  It helps clarify whether there is a risk for blood thinning from taking fish oils.  This information may help individuals who are taking blood thinning medications and considering adding fish oils to their daily diet.

  • Dave

    10/28/2009 2:22:01 AM |


    Without a doubt, your cessation of bruising was due to vitamin k2. I routinely take nattokinase, large doses of fish oil, curcumin, and other blood thinning agents, and if I don't take vitamin K2, I will begin bruising. (I also take high doses of Vitamin D). When I take K2, I have absolutely no bruising.

    Vitamin K2 has many clinical trials showing that it helps endothelium  integrity and elasticity.

    Also, grapeseed extract and pine bark extract (specifically oligomeric proanthcyanins) has the same beneficial effect.

  • Catherine

    10/28/2009 4:41:41 PM |


    That's really interesting! There is a lot of research on K2's effect on strengthening weak bones. Bone fractures go down considerably when high doses of K2 are used (Japan is using K2 as osteoporosis treatment) BUT studies show it needs to be in conjunction with adequate calcium and Vitamin D---they work synergistically for bone strength.  So it makes sense that K2 and D could do the same with strengthening fragile capillaries. I am also taking the M7 natto form.

  • Catherine

    10/29/2009 12:01:36 AM |


    Thanks for sharing your experience with this, you've really confirmed it now for me.  I can't believe I have suffered with this for most of my life with no answers (tried high dose Vit C, grape seed, etc) and now within months on K2, there's no bruising and I can tolerate fish oil. Hope my bones are responding this well!
    This blog is so helpful....

  • Mina

    10/29/2009 12:21:31 PM |

    Thanks for posting this. The question recently came up in our office. I like your assertion that omega-3s restore the blood to normal and remove abnormal clotting. And to comment on a post above, our dog has a beautifully shiny coat and takes 2 pure EPA capsules each day!

  • Term papers

    1/26/2010 3:40:08 PM |

    I have enjoyed reading That During nearly 3 years of observation, there was no excess of bleeding events in the group taking fish oil. (In fact, the group not taking fish oil had more bleeding events, though the difference fell short of achieving statistical significance.

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    8/23/2010 6:41:39 PM |

    I've been drinking fish oil for many year and I don't have any chance in my body people use to said me that but I think it is just a rumor.

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  • moseley2010

    12/7/2010 2:37:16 AM |

    I haven't heard of this problem
    fish oil supplements. But now we know what to tell them when this sort of concern comes up. Fish oil or Omega-3 is really beneficial to health. It's just important that it comes from clean waters.

  • Jack

    3/12/2013 7:03:38 PM |

    What is an appropriate dose of fish oil for someone taking coumadin?

  • dorange

    6/15/2014 3:53:03 PM |

    Dr. Davis, when  person is taking Tamoxifen...
    (1) is it safe to take vitamin k2 or K1?
    (2) will fish oil have a role in preventing blood clots?