Oil extracted from the livers of cod rather than from body tissue. It supplies EPA and DHA like other fish oils, plus substantial preformed vitamin A and vitamin D, because those vitamins are stored in the liver.
Cod liver oil is not a fish oil with extras — it is a vitamin A supplement that also contains omega-3, and that reversal is the whole safety story. A teaspoon commonly supplies on the order of 1,000–1,350 µg RAE of preformed retinol, close to or above the adult recommended intake, against a Tolerable Upper Intake Level of 3,000 µg RAE for preformed vitamin A. Anyone who scales it up to reach fish-oil trial doses of EPA+DHA will reach hypervitaminosis A territory long before they reach a therapeutic omega-3 dose, and vitamin A content varies widely between brands and is often not stated clearly.
Should state vitamin A and vitamin D per serving as well as EPA and DHA; many products do not state retinol clearly — a product without this marker may not resemble what was studied.
Regulation: Sold as a dietary supplement in the US with no pre-market efficacy or safety review. Several European health bodies specifically advise pregnant women to avoid cod liver oil because of its preformed retinol content, and advise against taking it alongside a multivitamin containing retinyl esters. Norway and other producing countries regulate vitamin A content in the product.
What the research used
The amounts, forms and durations published trials of Cod liver oil actually administered.
These are the amounts published trials administered — not a recommendation, and not a dose to copy. Trial participants were screened, supervised and given a characterised preparation. What is in a retail bottle is frequently not what was studied, and the amount that suits one person is not general advice.
Studied for Traditional daily supplementation
5–10 ml
Limited
Liquid oil
A 5 ml teaspoon supplies roughly 0.5–1 g of EPA+DHA together with roughly 1,000–1,350 µg RAE of preformed vitamin A and 10–12 µg of vitamin D, but these figures differ several-fold between products. The vitamin A is the limiting ingredient, not the omega-3.
Studied for Rickets prevention in historical public health use
5 ml
Moderate
Liquid oil, daily
Cod liver oil was the intervention that established the vitamin D and rickets link in the early twentieth century. Modern practice uses measured vitamin D rather than an oil with a variable and unlabelled retinol load.
This is general information about food, not medical advice. It cannot diagnose, treat or replace guidance from a clinician who knows your history.
When it was taken
Each window says whether a trial compared it against an alternative, or whether it is convention and pharmacology. Most timing advice is the second kind.
Each timing below was actually compared against an alternative in a trial, which is unusual.
🍽️With food
Moderate
Taken with a meal containing fat, both because the omega-3s and the fat-soluble vitamins are absorbed through the lipid route and because the oil is better tolerated that way. The fat dependence of fat-soluble vitamin absorption is established pharmacology.
Timing was tested. A trial compared this window against another and reported a difference.
The evidence in depth
Best supported: Moderate
What each association actually rests on, and the papers behind it. A rating describes the state of the research, not how promising the supplement sounds. How we rate evidence.
The EPA and DHA in cod liver oil lower triglycerides by the same mechanism as any other marine oil, but the retinol content makes it impractical to reach the doses used in triglyceride trials. The lipid evidence belongs to EPA+DHA, not to this delivery format.
Moderate evidence. Several human studies point the same way, with real caveats.
A small number of trials, mainly in rheumatoid arthritis, report reduced use of non-steroidal anti-inflammatory drugs on cod liver oil. The trials are few and small, and the effect is generally attributed to the EPA and DHA rather than to anything specific to liver oil.
Limited evidence. Early human work, or mostly lab and animal research.
There is not enough human research to say cod liver oil changes infection rates. Observational studies in Norwegian cohorts have linked use to fewer upper respiratory symptoms, but these are confounded by the kind of person who takes cod liver oil daily, and the vitamin D content complicates attribution.
Not enough evidence. Not enough human research to say anything useful yet.
Randomised controlled trial · Zhonghua yu fang yi xue za zhi [Chinese journal of preventive medicine] · 2011 · PMID 21624240
Deep research mode adds each paper’s own conclusion and stated limitations.
Things to know
Interactions, contraindications and dose limits, most serious first. Concentrated extracts interact with medication in ways the whole plant does not, so this section carries more weight here than it does on a food page.
Too much
Preformed vitamin A and hypervitaminosis A
Cod liver oil delivers retinol, not beta-carotene, so intake is not regulated by conversion and the liver stores what it cannot use. Chronic excess causes headache, bone and joint pain, hair loss, dry skin, raised intracranial pressure and liver injury, and has been associated with reduced bone mineral density and higher hip fracture risk in observational data. The adult upper limit for preformed vitamin A is 3,000 µg RAE/day, which several teaspoons of cod liver oil will exceed on their own.
Worth knowing
Do not stack with other retinol sources
Interacts withRetinoid drugs such as isotretinoin and acitretin, retinol-containing multivitamins
Retinol from cod liver oil adds to retinol from multivitamins, fortified foods, liver and any oral retinoid medication. Checking whether a multivitamin supplies vitamin A as retinyl palmitate or as beta-carotene is the practical step, because the two are not equivalent for toxicity.
Avoid
Pregnancy
Preformed vitamin A is a known human teratogen at high intake, and several national health bodies advise avoiding cod liver oil in pregnancy for that reason. Where long-chain omega-3 is wanted in pregnancy, algal DHA or a purified fish oil carries no retinol load.
Interaction
Bleeding risk and anticoagulants
Interacts withWarfarin, direct oral anticoagulants, antiplatelet drugs
The same platelet effects as other marine oils apply, and cod liver oil also contributes vitamin D, which is relevant to calcium handling. Prescribers should know about it before surgery or alongside anticoagulant therapy.
Worth knowing
Oxidation and unverified "fermented" products
Liver oils oxidise like any other marine oil, and independent testing of fermented cod liver oil products has reported high oxidation markers and vitamin contents that differed substantially from the label. Fermentation of an oil is not an established purification process and does not imply higher quality.
In the food catalogue
The whole-food form, where one exists. The evidence for a concentrated extract rarely transfers to the food, or the other way round — these are cross-links, not equivalents.