Nordic populations have used cod liver oil for centuries, originally to help protect against the harsh cold of Scandinavian winters. By 1930, scientists had worked out that cod liver oil could prevent rickets, and it became a household staple for families wanting to ward off vitamin D deficiency. These days, cod liver oil remains popular, though the evidence for some of its supposed benefits is stronger than for others.

What is cod liver oil? How does it differ from fish oil?

Cod liver oil comes specifically from the liver of Atlantic cod (Gadus morhua). The main distinction from regular fish oil is that cod liver oil contains substantial amounts of vitamin A and vitamin D alongside its omega-3 fatty acids (EPA and DHA).

Fish oil is extracted from the flesh of fatty fish like salmon, mackerel, and sardines. It provides omega-3s but typically contains little to no vitamins A or D. This makes cod liver oil something of a two-in-one supplement, though it also means you need to watch your total vitamin A and D intake if you’re taking other supplements.

Why vitamin A matters

Vitamin A plays roles in immune function, bone growth, night vision, cell development, and reproductive health. Deficiency causes night blindness and can impair immune function, though outright deficiency is uncommon in developed countries.

Why vitamin D matters

Beyond preventing rickets, vitamin D insufficiency has been linked to increased risks of type 1 diabetes, cardiovascular disease, certain cancers, cognitive decline, depression, pregnancy complications, and autoimmune conditions. The strength of these associations varies considerably, and correlation does not equal causation. Still, maintaining adequate vitamin D levels seems prudent given its role in calcium absorption and bone health.

What does the research actually show?

I’ve gone through the clinical evidence for cod liver oil’s most commonly claimed benefits. Some have decent support; others are more speculative.

1. Cod liver oil may help reduce anti-inflammatory medication use in rheumatoid arthritis

Rheumatoid arthritis is an autoimmune condition affecting roughly 1% of adults. The immune system attacks the joints, causing chronic inflammation, pain, and progressive joint damage. Standard treatment typically involves non-steroidal anti-inflammatory drugs (NSAIDs), disease-modifying drugs, and sometimes biologics.

A double-blind, placebo-controlled randomised trial followed 97 patients with rheumatoid arthritis for nine months [1]. Participants took either 10 grams of cod liver oil daily or a placebo. The cod liver oil group was able to reduce their NSAID use compared to the placebo group, though clinical measures of disease activity did not show clear differences between groups.

My honest assessment: this suggests cod liver oil might allow some people with rheumatoid arthritis to take fewer painkillers, but it’s not going to replace proper disease-modifying treatment. The sample size was small, and we need more research before drawing firm conclusions. If you have rheumatoid arthritis and want to try cod liver oil, discuss it with your rheumatologist rather than adjusting your medications on your own.

2. Maternal cod liver oil use during pregnancy may reduce type 1 diabetes risk in offspring

Type 1 diabetes results from immune-mediated destruction of insulin-producing beta cells in the pancreas. While it can develop at any age, it’s one of the most common chronic conditions in children, with peaks around ages 5-7 and again during adolescence.

A population-based case-control study compared 85 children with type 1 diabetes to 1,071 controls [2]. Mothers who took cod liver oil during pregnancy had offspring with a substantially lower risk of developing type 1 diabetes, with the risk reduction estimated at around 70%.

The catch? This was an observational study, not a randomised trial. Women who take cod liver oil during pregnancy might differ from those who don’t in ways that independently affect diabetes risk. Perhaps they’re more health-conscious generally, have better diets, or have higher socioeconomic status. The researchers tried to adjust for confounding factors, but you can never fully eliminate this concern in observational research.

The biological plausibility is reasonable, though. Both vitamin D and omega-3 fatty acids have immunomodulatory effects, and adequate vitamin D during pregnancy and early life might help prevent the autoimmune process that leads to type 1 diabetes. But “might” is doing a lot of work in that sentence.

3. Cod liver oil users report fewer depressive symptoms

Depression affects millions worldwide, with lifetime prevalence estimates of around 5.5% in women and 3.2% in men. The condition involves persistent low mood, feelings of hopelessness, reduced interest in activities, and can include sleep disturbances, appetite changes, and difficulty concentrating.

A large cross-sectional study of 21,835 participants found that people who regularly took cod liver oil reported fewer depressive symptoms than non-users [3]. This association held up after adjusting for age, sex, smoking, coffee and alcohol consumption, physical activity, and education level.

I should be clear about what this study does and doesn’t tell us. Cross-sectional studies can identify associations but cannot establish causation. We don’t know whether:

Cod liver oil actually reduces depression
People with depression are less likely to take supplements
Some third factor (like general health-consciousness) influences both supplement use and mood

The omega-3 fatty acids in cod liver oil have biological plausibility for mood effects, given their role in brain cell membrane composition. Vitamin D deficiency has also been linked to depression. But we would need randomised controlled trials specifically testing cod liver oil for depression to know if the association is causal.

4. Cod liver oil may support eye health in glaucoma

Glaucoma involves progressive damage to the optic nerve, typically associated with elevated intraocular pressure. It affects over 70 million people worldwide and is the leading cause of irreversible blindness globally [4].

The rationale for cod liver oil in glaucoma involves both its vitamin A and omega-3 content. Vitamin A is essential for normal vision and helps maintain the health of the conjunctiva and cornea. It also has antioxidant properties, and oxidative stress is implicated in glaucoma pathophysiology.

Omega-3 fatty acids may help reduce intraocular pressure and improve blood flow to the eye. Some research suggests they have neuroprotective effects that could benefit the optic nerve.

However, I should be upfront: I couldn’t find randomised controlled trials specifically testing cod liver oil for glaucoma prevention or treatment. The theoretical benefits are based on the known effects of its component nutrients rather than direct evidence for the supplement itself. If you have glaucoma or are at risk, standard medical treatment and monitoring are far more important than any supplement.

5. Cod liver oil may reduce coronary heart disease mortality

Coronary heart disease develops when atherosclerosis narrows the heart’s major blood vessels, reducing oxygen delivery to the heart muscle. Symptoms can include chest pain, shortness of breath, and fatigue. Complete blockage of a coronary artery causes a heart attack.

A large prospective cohort study followed 22,035 participants for 22 years [5]. Regular users of omega-3 supplements (primarily cod liver oil) had a 26% lower risk of dying from coronary heart disease compared to non-users. The benefit was most pronounced in people who ate little fish and were around 60 years old.

This is a reasonably well-conducted observational study with a long follow-up period. The 26% reduction in mortality is meaningful if it’s real. However, the same caveats about observational research apply: we can’t be certain the supplement use itself caused the benefit rather than some associated factor.

The omega-3 fatty acids in fish and fish oil have well-established effects on triglycerides and may have anti-arrhythmic and anti-inflammatory properties. Randomised trials of omega-3 supplements for cardiovascular outcomes have shown mixed results, though some large trials have been positive.

Side effects of cod liver oil

Cod liver oil is generally well-tolerated when taken at recommended doses. Reported side effects tend to be mild and include:

Fishy aftertaste or burping
Bad breath with a fishy odour
Heartburn or indigestion
Nausea
Loose stools

Taking cod liver oil with food often reduces these gastrointestinal side effects. Keeping the oil refrigerated can also help reduce the fishy taste.

Vitamin A toxicity risk

The vitamin A content of cod liver oil creates a genuine safety concern that doesn’t apply to regular fish oil. Taking more than the recommended dose can lead to vitamin A accumulation, since it’s fat-soluble and stored in the liver.

Symptoms of vitamin A excess include:

Nausea and vomiting
Headache
Dizziness
Fatigue
Dry skin and skin peeling
In severe cases, liver damage and increased intracranial pressure

The tolerable upper intake level for vitamin A is 10,000 IU (3,000 mcg retinol activity equivalents) daily for adults [6]. If you’re taking other supplements containing vitamin A, or eating a lot of liver, you need to account for all sources.

Vitamin D toxicity risk

Very high doses of vitamin D can also cause toxicity, leading to hypercalcaemia (excess calcium in the blood). This can result in:

Nausea and vomiting
Weakness and fatigue
Bone pain
Kidney stones
In severe cases, kidney damage and calcification of blood vessels

The tolerable upper intake level for vitamin D is 4,000 IU (100 mcg) daily for adults [7]. Toxicity typically requires much higher doses over extended periods, but it’s worth being aware of your total intake from all sources.

Safety precautions (3 contraindications)

1. Pregnancy and breastfeeding require caution

While the study mentioned earlier suggested maternal cod liver oil might reduce offspring diabetes risk, pregnant women need to be careful about vitamin A intake. Excessive vitamin A during pregnancy can cause birth defects, particularly in the first trimester.

The NHS advises pregnant women to avoid supplements containing vitamin A (including cod liver oil) unless specifically recommended by a doctor [8]. If you’re pregnant and want to take omega-3 supplements, regular fish oil (which doesn’t contain vitamin A) or specific pregnancy formulations are safer options.

Breastfeeding women should also be cautious, as vitamin A passes into breast milk. Discuss supplementation with your healthcare provider.

2. Blood pressure medications and cod liver oil may interact

Cod liver oil’s omega-3 fatty acids have a mild blood pressure-lowering effect. If you’re already taking antihypertensive medications, combining them with cod liver oil could potentially lower your blood pressure too much.

Medications to be aware of include:

ACE inhibitors (captopril, enalapril, lisinopril)
Angiotensin receptor blockers (losartan, valsartan)
Calcium channel blockers (diltiazem, amlodipine)
Diuretics (hydrochlorothiazide, furosemide)
Beta-blockers (metoprolol, atenolol)

This doesn’t mean you absolutely cannot take cod liver oil with blood pressure medications, but you should discuss it with your doctor and monitor your blood pressure, particularly when starting supplementation.

3. Anticoagulant medications require caution

Omega-3 fatty acids have mild blood-thinning effects. If you’re taking anticoagulant or antiplatelet medications, adding cod liver oil could theoretically increase bleeding risk.

Relevant medications include:

Warfarin
Heparin
Aspirin (at antiplatelet doses)
Clopidogrel
Direct oral anticoagulants (rivaroxaban, apixaban, dabigatran)

Stop taking cod liver oil at least two weeks before scheduled surgery, and inform your surgeon and anaesthetist about any supplements you take.

How much cod liver oil should you take?

There’s no official recommended dose for cod liver oil specifically. Most products suggest 1-2 teaspoons of liquid or 1-3 capsules daily. The key constraints are the vitamin A and D content rather than the omega-3s.

Check the label of your specific product for vitamin A and D amounts per serving, and ensure your total daily intake from all sources stays within tolerable upper limits:

Vitamin A: below 10,000 IU (3,000 mcg RAE) daily
Vitamin D: below 4,000 IU (100 mcg) daily

If you’re taking a multivitamin or other supplements containing these vitamins, factor that into your calculations.

References

Galarraga B, Ho M, Youssef HM, et al. Cod liver oil (n-3 fatty acids) as an non-steroidal anti-inflammatory drug sparing agent in rheumatoid arthritis. Rheumatology (Oxford). 2008;47(5):665-669. https://pubmed.ncbi.nlm.nih.gov/18362100/
Stene LC, Ulriksen J, Magnus P, Joner G. Use of cod liver oil during pregnancy associated with lower risk of Type I diabetes in the offspring. Diabetologia. 2000;43(9):1093-1098. https://pubmed.ncbi.nlm.nih.gov/11043854/
Raeder MB, Steen VM, Vollset SE, Bjelland I. Associations between cod liver oil use and symptoms of depression: the Hordaland Health Study. J Affect Disord. 2007;101(1-3):245-249. https://pubmed.ncbi.nlm.nih.gov/17184843/
Kang JH, Pasquale LR, Willett WC, et al. Dietary fat consumption and primary open-angle glaucoma. Am J Clin Nutr. 2004;79(5):755-764. https://pubmed.ncbi.nlm.nih.gov/15113712/
Lindberg M, Saltvedt I, Sletvold O, Bjerve KS. Long-chain n-3 fatty acids and mortality in elderly patients. Am J Clin Nutr. 2008;88(3):722-729. https://pubmed.ncbi.nlm.nih.gov/18779289/
NHS. Vitamin A. https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-a/
NHS. Vitamin D. https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-d/
NHS. Vitamins, supplements and nutrition in pregnancy. https://www.nhs.uk/pregnancy/keeping-well/vitamins-supplements-and-nutrition/