Can you take too much fish oil?
At high pharmaceutical doses of fish oil (1,800 to 4,000 mg per day) the risk of an irregular heartbeat rises measurably; at the amounts most people take daily, the risks are small and there are instead signs of modest heart protection.
Fish oil is not dangerous at the amounts most people take daily, but higher doses carry real risks. The best-known concern is the effect on heart rhythm: pharmaceutical doses of omega-3 of 1,800 to 4,000 mg per day raise the risk of atrial fibrillation (an irregular heartbeat) in a dose-dependent way. At the highest doses, the relative risk is about 50% higher. In absolute terms: 4.0% of people on higher doses developed atrial fibrillation, compared with 3.3% across lower-dose groups. Lower dietary intake, around 650 mg per day, is actually linked to a lower risk of that condition.
The bleeding risk is more nuanced. With ordinary omega-3 supplements, no meaningful increase in bleeding risk was found. Only with high-dose, purified EPA (a specific fish oil component in pharmaceutical form) did the bleeding risk rise by about 50% in relative terms. The absolute difference is modest: 0.6 percentage points more than in the comparison group. Brain bleeds and gastrointestinal bleeds did not increase in a meaningful way across all doses and types.
For the heart there are also benefits. Omega-3 supplements lower blood fats (triglycerides) by about 15% and probably slightly reduce the risk of dying from heart disease and having a heart attack. The effect on strokes and overall risk of death is small or absent.
In short: at usual doses of fish oil the risks are small. Anyone taking pharmaceutical doses would be wise to discuss this with a doctor, especially if they have an elevated risk of heart rhythm problems.
This answer is based on several large bodies of research, including combined analyses of tens of studies involving hundreds of thousands of participants. The atrial fibrillation risk at high doses is one of the most robustly supported findings, confirmed in multiple independent analyses. The bleeding risk at ordinary doses is also well studied; a signal was found only with purely high-dose EPA. The evidence for heart protection exists but is less strong: the benefit is small and does not hold for all outcomes.