Omega-3 fish oil

What it is

Omega-3 fatty acids EPA and DHA are fats found mainly in oily fish such as salmon, mackerel, sardines and herring. Supplements come as fish oil, krill oil, cod liver oil (which also supplies vitamins A and D) or algal oil, which is vegan and made from microalgae.

People take them for heart health, brain health, joints, mood and general inflammation. UK advice is to eat at least one portion of oily fish a week (check current NHS guidance), and supplements are an option for people who do not.

Evidence differs by use. High-dose purified prescription forms have the strongest data, while off-the-shelf capsules in healthy people have often shown little effect on heart attacks and strokes.

How it works

EPA and DHA are built into cell membranes and used to make signalling molecules that influence inflammation, blood clotting and fat metabolism. They lower triglycerides by reducing the liver's production of fat particles. DHA is also a major structural fat in the brain and retina.

What the evidence says

• Strong Lowering high triglycerides
  Reliable, dose-dependent reduction at prescription-level doses (grams a day), usually under medical supervision.
• Limited Cardiovascular events in the general population
  Large trials of standard doses in people without disease mostly show little or no reduction; some higher-risk groups may benefit, particularly with purified EPA under prescription.
• Moderate Blood pressure
  Small average reductions, a few mmHg, mainly in people with raised blood pressure.
• Moderate Joint pain in rheumatoid arthritis
  Modest reduction in stiffness and pain at higher doses, as an add-on to treatment.
• Weak Depression and cognition
  Some benefit with EPA-dominant formulas for depression in certain trials, but results are inconsistent; little evidence of a cognitive boost in healthy adults.
• Limited Infant brain and eye development (DHA in pregnancy)
  Eating fish is recommended; supplement benefits are modest and mixed.

Evidence ratings are our plain-English summary of how much good human research exists, not a promise of results. How we rate evidence.

Dosage and timing

• For general use, 250 to 500 mg a day of combined EPA plus DHA is a common target, or about one to two portions of oily fish a week.
• Doses above about 1 g a day of EPA plus DHA are worth discussing with a clinician, especially with heart rhythm or bleeding issues; check current guidance for upper limits.
• Check the EPA plus DHA content per capsule, which is usually well below the "1,000 mg fish oil" headline number.
• Take with food containing fat; split doses if you get reflux or fishy burps.

Forms and what to look for

• Triglyceride and re-esterified forms are generally better absorbed than ethyl esters, especially with a low-fat meal.
• Krill oil contains phospholipid-bound omega-3 in lower amounts; whether it is more bioavailable is debated, and you need more capsules for the same EPA plus DHA.
• Algal oil provides DHA (and sometimes EPA) for vegetarians and vegans, and avoids fish contaminants.
• Cod liver oil contains vitamins A and D; avoid stacking it with other vitamin A sources, especially in pregnancy.

What to expect, and when

• Triglyceride reductions appear within 4 to 8 weeks at adequate doses.
• Blood omega-3 levels rise over weeks; other effects take 3 months or more.
• You are unlikely to notice anything day to day.

Side effects

• Fishy aftertaste, burps, nausea or reflux are common.
• Loose stools at higher doses.
• Slightly increased bleeding tendency, such as nosebleeds or bruising, at higher doses.
• Higher doses are associated with a small increased risk of atrial fibrillation in some trials.

Interactions

• Blood thinners (warfarin, apixaban, aspirin, clopidogrel): higher doses might add to bleeding risk; tell your prescriber.
• Blood pressure medicines: may add slightly to the lowering effect.
• Other supplements with blood-thinning effects, such as high-dose vitamin E, ginkgo or garlic, may add up in theory.
• Orlistat can reduce absorption of fat-soluble nutrients including omega-3s.

Who should avoid it

• People with an irregular heartbeat or atrial fibrillation, or at risk of it, should take advice before using high doses.
• Anyone with a bleeding disorder, or about to have surgery, should check with their team.
• Fish or shellfish allergy for fish-based products; algal oil is an alternative.
• Pregnant or breastfeeding women should avoid cod liver oil (vitamin A) and check current NHS advice on supplement choice.

Risks and safety

• Oxidation: fish oil goes rancid with heat, light and age, and oxidised oil tastes and smells off; some independent tests have found oxidation above recommended limits in off-the-shelf products.
• Contaminants such as mercury and PCBs are usually low in purified oils, but quality varies.
• Very high doses increase the risk of bleeding and rhythm problems.
• Labels on EPA plus DHA content can be inaccurate in cheaper brands.

Buying well: quality and testing

• Check the EPA and DHA amounts per serving, not just total fish oil.
• Look for third-party testing (IFOS, Informed Sport, NSF, USP) for purity and oxidation.
• Choose products with added antioxidants such as vitamin E or rosemary extract, and a recent best-before date.
• If it smells strongly rancid or tastes foul, throw it away; keep opened bottles refrigerated.

Bottom line

Omega-3 supplements make most sense if you rarely eat oily fish, or for a clinician-directed purpose like high triglycerides. For healthy people, eating oily fish weekly is probably better value than expensive capsules. If you take blood thinners or have heart rhythm problems, speak to your prescriber before starting, and buy a tested, fresh product.