What is the difference between fish oil and cod liver oil?

Fish oil is extracted from the body of oily fish (such as sardines, anchovies, and mackerel) and is high in EPA and DHA with minimal vitamins. Cod liver oil is extracted from the liver of cod and contains EPA and DHA but also significant quantities of vitamins A and D. For omega-3 benefits, either source is acceptable. However, because cod liver oil contains preformed vitamin A (retinol), high doses can reach or exceed safe upper limits — the NHS advises pregnant women in particular not to take cod liver oil or any supplement containing vitamin A. If your primary goal is omega-3 intake, standard fish oil allows higher EPA+DHA doses without the vitamin A constraint.

What are vegan alternatives to fish oil?

Algae-derived omega-3 supplements provide EPA and DHA directly — without the fish. Oily fish accumulate their omega-3 content by eating marine algae and microalgae; algae-based supplements go directly to that source. Algae oil is nutritionally equivalent to fish oil EPA and DHA and has the advantage of being free from the heavy metal and PCB contamination risks associated with fish. Bioavailability studies show that algae-sourced DHA raises plasma DHA comparably to fish oil. If you follow a plant-based diet, look for algae-based supplements that declare EPA and DHA content separately, not simply ALA. Brands including Testa, Vegetology, and Bare Biology offer certified algae omega-3 in the UK.

When is the best time to take fish oil?

Fish oil should be taken with a meal that contains fat. Omega-3 fatty acids are fat-soluble; co-ingestion with dietary fat significantly improves absorption — studies show up to a 50% increase in plasma EPA+DHA levels when taken with a fat-containing meal versus fasted. This is particularly relevant for ethyl ester-form products, which are more dependent on the fat co-ingestion effect. There is no evidence that morning versus evening timing makes a meaningful difference to outcomes; consistency of daily intake matters more than precise timing.

Should fish oil capsules be refrigerated?

Sealed, unopened capsules do not require refrigeration if stored below 25°C, away from direct light and heat. Once opened, refrigeration after each use will slow oxidative degradation, particularly for liquid fish oil products. Capsules are generally more oxidatively stable than liquids due to reduced surface area exposure to oxygen. If your capsules produce a strong, rancid fish odour, this is a sign of oxidation and you should consider switching to a product with verified low oxidation markers (IFOS certified) or freshening the bottle from a different supplier.

How much EPA and DHA do I actually need per day?

The EFSA approved health claim for maintaining normal cardiac function requires 250 mg/day combined EPA+DHA. For triglyceride reduction, EFSA and the American Heart Association recognise 3–4 g/day of combined EPA+DHA. The NHS does not set a formal supplement dosage recommendation but endorses eating oily fish twice weekly, which delivers approximately 1.5–3 g EPA+DHA per week. For general health maintenance in someone who does not eat oily fish, 1–2 g EPA+DHA per day is a practical and evidence-aligned target. Always check the EPA+DHA content on the label rather than the total fish oil weight — a 1,000 mg capsule may deliver as little as 300 mg EPA+DHA.

Can fish oil interact with blood thinners or other medications?

Yes. At doses of 3 g/day or more, omega-3 fatty acids extend bleeding time and may potentiate the effects of anticoagulant and antiplatelet medications including warfarin, aspirin, and clopidogrel. EFSA considers combined EPA+DHA up to 5 g/day safe for healthy adults, but those on anticoagulant therapy should consult their GP or pharmacist before exceeding 1 g/day. The atrial fibrillation risk identified at higher doses (≥3 g/day) in clinical trials is also relevant for individuals with a history of arrhythmia. At standard supplemental doses of 1 g/day, interaction risk is low for most people.

Is EPA or DHA more important — should I care about the ratio?

The importance of the ratio depends on your health goal. For cardiovascular outcomes and mood support, the clinical trial evidence favours EPA-enriched formulations. The meta-analysis by Khan et al. (2021, PMID: 34505026) found that EPA monotherapy demonstrated significantly greater cardiovascular risk reductions than combined EPA+DHA products. For brain structure, neurological health, retinal function, and pregnancy or infant development, DHA is the priority. General-purpose fish oil supplements (roughly 3:2 EPA:DHA ratio) are a reasonable starting point for most adults. If you are specifically targeting mood or cardiovascular outcomes, an EPA-dominant product (≥60% EPA) is better supported by the available evidence.

What does IFOS certification mean and why does it matter?

IFOS (International Fish Oil Standards) is an independent testing programme operated by Nutrasource in Canada. Products submitted for IFOS testing are assessed against benchmarks for oxidation (peroxide value, anisidine value, and TOTOX score), concentration accuracy (actual EPA+DHA versus labelled), heavy metals (mercury, lead, arsenic, cadmium), PCBs, dioxins, and furans. A five-star IFOS rating is the highest achievable and indicates the product met or exceeded all quality criteria at the time of testing. Given research showing that a substantial proportion of commercially available fish oil supplements exceed oxidation thresholds (PMID: 33382790), IFOS certification is currently the most reliable proxy for product quality available to UK consumers without laboratory access.

Omega-3 Fish Oil: What the Evidence Actually Says — A UK Buyer's Guide

Nutripedia Research Team14 April 2026

Omega-3 fish oil is one of the most widely purchased supplements in the UK, yet the clinical picture is more nuanced than the marketing suggests. This guide reviews the major trials, explains EPA vs DHA, flags the rancidity problem, and helps you choose a product that delivers what it promises.

Not medical advice

Nutripedia summarises published peer-reviewed research. This content is for informational purposes only and is not a substitute for professional medical advice. Product mentions are not endorsements.

What Are Omega-3 Fatty Acids? EPA, DHA, and ALA Explained

Omega-3 fatty acids are a family of long-chain polyunsaturated fats (PUFAs) considered essential because the human body cannot synthesise them in adequate quantities. The three main forms relevant to supplementation are eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), and alpha-linolenic acid (ALA). EPA (20 carbons, 5 double bonds) is primarily anti-inflammatory. It competes with arachidonic acid in the eicosanoid cascade, reducing the production of pro-inflammatory prostaglandins and leukotrienes. It is the form most associated with cardiovascular and mood benefits in clinical trials. DHA (22 carbons, 6 double bonds) is the dominant structural fatty acid in the brain, retina, and cell membranes of the nervous system. Approximately 97% of the omega-3 content of the brain is DHA. It is critical during foetal development and infancy; EFSA has approved the health claim that DHA maternal intake contributes to normal brain development of the foetus and breastfed infants. ALA (18 carbons, 3 double bonds) is the plant-based omega-3, found in flaxseed, chia seeds, walnuts, and rapeseed oil. Critically, human conversion of ALA to EPA is inefficient (approximately 5–8%) and to DHA negligible (<0.1%). Relying on ALA-rich foods alone does not reliably maintain EPA and DHA tissue levels. Vegans and vegetarians should therefore consider algae-derived EPA/DHA supplements — the original dietary source that oily fish accumulate their omega-3s from. Fish oil supplements contain EPA and DHA directly. The NHS recommends eating at least two portions of fish per week, including one portion of oily fish (salmon, mackerel, sardines, herring), which provides approximately 1.5–3 g EPA+DHA. For those who do not eat oily fish, supplementation provides a practical alternative. EFSA-approved claims recognise that 250 mg/day EPA+DHA maintains normal cardiac function, and that 3 g/day may help maintain normal blood triglyceride levels.

Our research is based on 79 peer-reviewed studies. View the full evidence database

Our Top Picks

All Picks — Ranked

Frequently Asked Questions

Sources

Nutripedia is an educational resource. Content is sourced from peer-reviewed studies and does not constitute medical advice. Product mentions are not endorsements. Consult a healthcare professional before starting any supplement.

Reviewed by

Archie Roberts

Founder, Nutripedia — ALDR Ltd

This page summarises published research from PubMed, NHS, EFSA, and SACN. It does not constitute medical advice; consult a qualified healthcare professional before changing any supplement regimen.

Last reviewed: 14 Apr 2026Methodology