Omega-3 fatty acids from fish oil have conclusively been shown to reduce risk for heart attack and stroke. They also have the capacity to "thin the blood."
What this means is that omega-3s slightly block the activation of blood platelets and inhibit their "clumping," two basic steps in clot formation. Omega-3s also inhibit production of arachidonic acid and thromboxane that trigger blood vessel constriction and inflammation. The blood clotting protein, fibrinogen, also an independent coronary risk factor when at high levels, is reduced by omega-3 fatty acids. Contrast these effects with that of obesity, sedentary behavior, and an unhealthy diet, all of which favor platelet activation, abnormal blood vessel constriction, and increased fibrinogen levels. But can fish oil cause excessive bleeding?
This question comes up frequently. In my cardiology clinic, we use moderate- to high-doses of omega-3 fatty acids to reduce triglycerides, correct inherited causes of heart disease like lipoprotein(a), and to reduce overall cardiovascular risk. But does this expose people to risk for bleeding?
An addition to the conversation comes from a Philadelphia experience reported in the American Journal of Cardiology. In this study, all 364 subjects in the study took both blood thinners, aspirin and Plavix (a platelet-inhibiting drug), mostly for coronary disease. (People typically receive this combination after a stent is implanted, for instance.) The mean dose of aspirin was 161 mg/day; mean dose of Plavix, 75 mg/day. 182 of the subjects, or half, were also taking fish oil at a mean dose 3000 mg (unspecified omega-3 content).
Over 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 Warfarin, with no increased bleeding risk when fish oil is added.
There are two situations, however, in which omega-3 fatty acids can contribute to blood thinning sufficient to be important. One is if you are deficient in vitamin K1, a vitamin crucial to forming blood clotting proteins; omega-3 fatty acids in the presence of vitamin K1 deficiency can cause an inadvertent excess risk of bleeding. For this reason, including plenty of vitamin K1-rich foods like broccoli, lettuces, green peppers, and other green-colored foods is important for this and other health benefits. The second situation is the intentional vitamin K1-"deficiency" induced by Warfarin; because omega-3s modestly reduce production of several clotting proteins (e.g., factors VII and X) that intersect with Warfarin's action, blood may thin more than without omega-3s present. For this reason, anyone adding omega-3 supplements while on Warfarin should simply work with their healthcare provider and have an INR checked within 1-2 weeks to assess any change and reassess INR if any dose change in omega-3s is made.
Outside of these special situations, the blood thinning effect of omega-3 fatty acids should be viewed as a benefit, not a risk.
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.