Omega-3 fats are among the best-studied nutrients there are, and for most people a sensible daily amount is a good idea. But "good for you" isn't the same as "more is always better." Dose, product quality and the medications you take can all change the picture. Here's when omega-3s can work against you, and what to do about it.
Should you stop taking omega-3 when you have a cold or flu?
This question comes from a 2009 study at the University of North Carolina. Mice fed a diet rich in fish oil for two weeks and then infected with influenza had 40% higher mortality and 70% higher lung viral load than mice on a control diet, even though their lungs were less inflamed. The researchers' explanation: the same anti-inflammatory effect that helps in chronic inflammation may blunt the immune response needed to clear an acute virus.
It's an interesting finding, but keep it in proportion. It was a mouse study, fish oil made up a large share of the animals' diet, and the result hasn't been shown in people, so on its own it isn't a reason to stop a normal daily dose when you get the sniffles. If you're seriously ill or in the hospital, ask your doctor to review everything you take, supplements included.
Can you take too much omega-3?
Yes, and heart rhythm is the clearest example. A meta-analysis of seven large cardiovascular trials, covering more than 81,000 people, found that marine omega-3 supplements were linked to a higher risk of atrial fibrillation, an irregular heart rhythm. The risk was greater in trials using more than 1 g a day (about 49% higher) than in trials using 1 g or less (about 12% higher).
Bleeding is the other worry people raise. A 2024 meta-analysis of 11 trials and more than 120,000 patients found no overall increase in bleeding with omega-3s, though high-dose purified EPA (a prescription product) carried a small extra risk. If you take blood thinners or antiplatelet drugs, have a bleeding disorder or have surgery coming up, talk to your doctor before taking omega-3s, and don't take high doses without medical advice.
Is rancid or under-dosed fish oil a problem?
Omega-3 fats are fragile and oxidize easily, and testing suggests many products on shelves aren't what their labels promise:
- In New Zealand, only 3 of 32 fish oil supplements contained at least the EPA and DHA stated on the label, and 83% exceeded the recommended peroxide level, a marker of oxidation.
- In North America, 50% of 171 omega-3 products exceeded voluntary recommended limits for oxidation markers, and another 18% were approaching them.
An oxidized oil tastes and smells fishy, and you may simply be getting less of the omega-3s you paid for. Buy from brands that test every batch and publish the results, store your supplement as the label directs, and throw it out if it smells rancid or is past its date.
What about mercury and other contaminants?
Fish can carry mercury and other pollutants from the ocean, which is why pregnant women are advised to choose low-mercury fish. Fish oils are refined to reduce contaminants, but quality varies by brand.
Algae sidesteps much of the issue. Fish get their omega-3s from microalgae in the first place, and DHA from algal oil is absorbed as well as DHA from cooked salmon. That's why Head & Heart is made from algae rather than fish: 500 mg of DHA per serving, with every batch third-party tested for heavy metals and contaminants. You can read our certificates of analysis too.
How to take omega-3 sensibly
- Stick to the dose on the label unless your doctor recommends otherwise.
- Check with your doctor first if you take blood thinners, have a heart rhythm problem, are pregnant or have surgery planned.
- Choose a product that is third-party tested and tells you how much EPA and DHA it contains.
- Remember food counts: oily fish, or algae if you're plant-based, plus ALA from flax and chia. Our guide to ALA vs. EPA vs. DHA explains the difference, and this look at the omega-3 industry covers quality in more depth.
