For elective surgery, doctors generally advise against taking fish oil supplements for a period of time before and after the operation to avoid the risk of dangerous bleeding. If you have surgery scheduled, you need to know what new research shows about that seemingly obvious advice.

Nearly 20 million Americans take fish oil supplements daily for the heart-benefiting omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Fish oil reduces risk for heart attack by decreasing the ability of blood to form clots. In fact, the American Heart Association (AHA) recommends fish oil for people with heart disease.

However, the same function that keeps blood clots from forming can increase the risk for unwanted bleeding—including life-threatening hemorrhage. (This effect was first noticed among Eskimos, who consume high amounts of fish oil from their diet.)

Although studies on fish oil and bleeding have been small and inconsistent, to be on the safe side doctors usually advise patients to stop fish oil before undergoing medical procedures where bleeding is likely, such as surgery. Now a new study led by researchers at University of Iowa is questioning that recommendation.

The new research comes from the Omega-3 Fatty Acids for Prevention of Postoperative Atrial Fibrillation (OPERA) study—a multinational trial that ran from 2010 through 2012 and looked at more than 1,500 heart-surgery patients, average age about 63. While the trial did not find that fish oil reduced incidence of atrial fibrillation, the researchers were able to use the data for a secondary analysis on bleeding risk.

For the study, the patients were given either fish oil or a placebo that did not contain EPA or DHA (olive oil) from two to five days before their surgeries until their discharge. The fish oil supplements contained about 7 g of combined EPA and DHA before surgery and about 2 g  on the day of surgery and after.

Results: Fish oil did not increase the risk of bleeding after heart surgery. In fact, patients in the fish oil group had less bleeding and were less likely to need a transfusion during or after surgery than the placebo group.

Further, the more fish oil the better. Within the fish oil group, patients who had the highest levels when their blood was tested just before surgery had a 60% to 70% lower risk of bleeding—and significantly less need for a blood transfusion—compared to patients with the lowest levels of fish oil.

Bottom Line

The fish oil used in the OPERA study was Lovaza, a high-dose prescription form of fish oil made by GlaxoSmithKline, who also funded the study. The amounts taken by the study patients were about twice the daily amount recommended by the AHA for those with heart disease. The study researchers do not suggest that people should increase the amount of fish oil they’re taking. However, they do conclude that the recommendation to delay surgery or stop fish oil supplements needs to be reconsidered.

Meanwhile, fish oil supplements of up to and including 3 g of combined EPA/DHA are generally safe for most people to take. Higher doses of fish oil omega-3s can be especially beneficial for people with high levels of triglycerides. Check with your doctor before taking more than 3 g per day. Check with your doctor before taking any amount if you are being treated for a chronic condition, especially high blood pressure.

Finally, remember that you can also get omega-3s from your diet and from algae-based supplements. AHA recommends eating fatty fish at least two times a week, which could be enough to get the heart-health benefits. Fatty fish like salmon, albacore tuna, herring, trout and sardines are high in omega-3 fatty acids.