Is Cod Liver Oil Safe? Risks and Side Effects

Cod liver oil is generally safe for most adults when taken at recommended doses, but it carries a few risks that ordinary fish oil does not, largely because it delivers high concentrations of preformed vitamin A alongside its omega-3 fatty acids and vitamin D. That vitamin A content is the single biggest reason to pay attention to dosing. Beyond that, cod liver oil can interact with blood-thinning medications, cause digestive discomfort, and in some cases contain low levels of environmental contaminants depending on the product’s sourcing and purification.

The Vitamin A Problem

Most fish oil supplements provide omega-3 fats and little else. Cod liver oil is different because it comes from the liver of Atlantic cod, an organ that stores fat-soluble vitamins. A standard teaspoon of cod liver oil can supply anywhere from 1,000 to 5,000 IU of preformed vitamin A (retinol), depending on the brand and formulation. Preformed vitamin A is not the same as beta-carotene from carrots and sweet potatoes. Your body converts beta-carotene into vitamin A only as needed, but preformed retinol is absorbed directly, and excess accumulates in the liver.

The European Food Safety Authority maintains an upper intake level for preformed vitamin A at 3,000 micrograms of retinol equivalents per day for adults, scaled down for children and infants based on body weight.

1PubMed Central. Scientific opinion on the tolerable upper intake level for preformed vitamin A and β-carotene

That sounds like a comfortable margin until you realize that a single tablespoon of some cod liver oil products can deliver close to that ceiling on its own. Add in a fortified cereal, a multivitamin, and a serving of liver pâté in the same day, and you can exceed the upper limit without trying especially hard.

Chronic vitamin A excess leads to a condition called hypervitaminosis A. Early symptoms are subtle and easy to dismiss: headaches, dry or peeling skin, fatigue, joint pain, and nausea. Over months or years, excessive intake can cause more serious liver damage, elevated intracranial pressure, and bone-density loss. The risk climbs if you take cod liver oil on top of a multivitamin that already contains retinol, which is why checking the combined vitamin A from all your supplements is a practical first step.

Pregnancy and Vitamin A Teratogenicity

Cod liver oil during pregnancy is a topic where the stakes are high and the margin for error is narrow. Vitamin A is essential for fetal development, but too much preformed retinol during early pregnancy can cause birth defects affecting the central nervous system, heart, and skeleton. The risk window is especially acute in the first 60 days after conception.

2PubMed Central. Vitamin A and Pregnancy: A Narrative Review

The tricky part is that both deficiency and excess are harmful. A pregnant woman needs enough vitamin A to support fetal growth but must stay well below the threshold where retinol starts causing malformations.

3PubMed Central. Vitamin A-Mediated Birth Defects: A Narrative Review

In developed countries where dietary intake is usually adequate, the greater practical danger is accidental excess rather than deficiency. Many prenatal vitamins already contain vitamin A, so stacking cod liver oil on top can push total intake uncomfortably high. If you are pregnant or planning to become pregnant, the safest approach is to discuss any cod liver oil use with your provider and to choose a standard omega-3 supplement (without added vitamin A) if omega-3s are the goal.

Bleeding and Blood Thinning

Omega-3 fatty acids, particularly EPA and DHA, reduce the stickiness of blood platelets. This is one of the reasons fish oils are considered heart-healthy, but it also means they can extend bleeding times. In one supplementation study, mean bleeding times increased by about 80%, and platelet aggregation dropped noticeably while participants were taking cod liver oil. Those effects reversed within about two weeks of stopping the supplement.

4PubMed. Alteration and recovery of bleeding times, platelet aggregation and fatty acid composition of individual phospholipids in platelets of human subjects receiving a supplement of cod-liver oil

A separate trial comparing cod liver oil with corn oil confirmed that cod liver oil was more potent at reducing collagen-induced platelet aggregation and the production of thromboxane, a compound that promotes clotting.

5Thrombosis and Haemostasis. The Effect of Cod Liver Oil and Corn Oil on Platelets and Vessel Wall in Man

For a healthy person taking a teaspoon of cod liver oil daily, this mild blood-thinning effect is unlikely to cause problems. But if you are on anticoagulant or antiplatelet medication like warfarin, aspirin, or clopidogrel, the combination could tip the balance toward excessive bleeding. Surgeons sometimes recommend stopping fish oil supplements a week or two before elective procedures for the same reason. If you are managing any condition that affects clotting, it is worth mentioning your cod liver oil use to whoever prescribes your medication.

Blood Pressure and Blood Sugar Interactions

Fish oils produce a modest blood-pressure-lowering effect, which is usually framed as a benefit. But the effect has a flip side if you are already on antihypertensive drugs: the combined drop can cause dizziness or lightheadedness, especially when standing up. Research on fish oil and hypertension notes that chronic supplementation produces a small blood pressure reduction in humans, and that high doses raise concerns about blood glucose control in people with diabetes and prolonged bleeding in surgical patients.

6PubMed. Fish oil, essential fatty acids, and hypertension

The blood sugar question is more nuanced than a simple warning would suggest. Animal research has shown that cod liver oil can improve lipid metabolism and partially control hyperglycemia in diabetic rats.

7PubMed. Effects of cod liver oil on tissue antioxidant pathways in normal and streptozotocin-diabetic rats

A human trial in women with gestational diabetes found that cod liver oil lowered fasting blood glucose, triglycerides, and markers of inflammation compared to a control group, though the primary endpoints did not reach significance between groups at every time point.

8PubMed Central. Cod-Liver Oil Improves Metabolic Indices and hs-CRP Levels in Gestational Diabetes Mellitus Patients: A Double-Blind Randomized Controlled Trial

In short, the evidence does not clearly show that cod liver oil worsens diabetes, but the older concern about high-dose fish oil and glucose control persists in clinical guidelines. If you manage diabetes with medication, monitoring blood sugar more closely when starting any fish oil supplement is a reasonable precaution.

Gastrointestinal Side Effects

The most common complaints from cod liver oil users are digestive: fishy burps, nausea, heartburn, loose stools, and an unpleasant aftertaste. These are the same side effects that plague all fish oil supplements, and they are more a nuisance than a danger. A large randomized trial of cod liver oil supplementation for preventing respiratory infections reported only low-grade side effects in both the cod liver oil and placebo groups, suggesting that serious adverse events are rare at standard doses.

9BMJ. Prevention of covid-19 and other acute respiratory infections with cod liver oil supplementation, a low dose vitamin D supplement: quadruple blinded, randomised placebo controlled trial

Taking cod liver oil with a meal rather than on an empty stomach helps reduce nausea and fishy reflux. Refrigerating or freezing liquid or capsule forms can also slow the release of oil in the stomach, which cuts down on burping. Some people find flavored liquid cod liver oil (often lemon or mint) more tolerable than unflavored varieties. If digestive side effects persist despite these adjustments, switching to a purified omega-3 concentrate that comes in enteric-coated capsules is a straightforward alternative.

Environmental Contaminants

Because cod liver oil comes from the liver, the organ most responsible for filtering toxins, there has been longstanding concern about contaminants like mercury, dioxins, and polychlorinated biphenyls (PCBs). The good news is that modern purification has made a substantial difference.

Mercury levels in commercially available cod liver oil supplements appear to be quite low. A study testing oils sold on the Polish market found that the average mercury concentration in cod liver oils was about 0.106 micrograms per kilogram, well below the acceptable standard for dietary supplements. The researchers concluded that these products do not pose a health threat from mercury.

10PubMed Central. Mercury Exposure from the Consumption of Dietary Supplements Containing Vegetable, Cod Liver, and Shark Liver Oils

Extraction technology also plays a role. Supercritical fluid extraction, a method using pressurized carbon dioxide, can eliminate 85 to 100% of mercury, 97 to 100% of cadmium, and all detectable lead from fish oil.

11Journal of Food Protection. Effect of Supercritical Fluid Extraction on the Reduction of Toxic Elements in Fish Oil Compared with Other Extraction Methods

Dioxins and PCBs are a trickier problem. These persistent organic pollutants accumulate in marine food chains and concentrate in the liver. Activated carbon treatment can remove nearly all dioxins and up to about 80% of dioxin-like PCBs from contaminated cod liver oil, though some types of PCBs resist removal due to their molecular shape.

12Journal of the American Oil Chemists’ Society. Removal of dioxins and PCB from fish oil by activated carbon and its influence on the nutritional quality of the oil

The concern is less about supplements from major brands, which typically undergo rigorous purification, and more about locally sourced or artisanal cod liver products. A retrospective analysis of cod liver products from 1972 to 2017 found that while industrially refined fish oils showed low contaminant levels, cod livers sourced from certain waters and consumed locally continued to contribute substantially to dietary intake of these pollutants.

13PubMed. A retrospective investigation into the occurrence and human exposure to polychlorinated naphthalenes (PCNs), dibenzo-p-dioxins and furans (PCDD/Fs) and PCBs through cod liver products (1972-2017)

If you want to minimize contaminant exposure, look for products that specifically state they have been molecularly distilled or purified using activated carbon treatment. Third-party testing certifications from organizations like the International Fish Oil Standards program provide an additional layer of assurance.

Rancidity and Oxidation

Fish oils are chemically fragile. The same polyunsaturated fatty acids that give omega-3s their health benefits are also highly susceptible to oxidation when exposed to heat, light, or oxygen. Rancid cod liver oil smells and tastes noticeably off, but mild oxidation can occur before the product smells obviously wrong. Secondary products of lipid oxidation can be absorbed by the body and may increase oxidative stress, with potential downstream effects on lipoprotein and platelet metabolism.

14Free Radical Biology and Medicine. Routes of formation and toxic consequences of lipid oxidation products in foods

Practically speaking, this means storage matters. Keep liquid cod liver oil refrigerated after opening and use it before the expiration date. If it tastes or smells strongly fishy or paint-like, toss it. Capsules are somewhat more protected from air exposure but should still be stored in a cool, dark place. The irony of taking a supplement for health while consuming oxidized fats that may cause harm is easy to avoid with basic handling care.

Fish Oil and Atrial Fibrillation Risk

A large prospective study using UK Biobank data found that regular use of fish oil supplements was associated with a slightly elevated risk of developing atrial fibrillation in people who were otherwise healthy. The adjusted hazard ratio was about 1.13, meaning roughly a 13% higher risk compared to non-users.

15BMJ Medicine. Regular use of fish oil supplements and course of cardiovascular diseases: prospective cohort study

This finding generated headlines, but some context is useful. The absolute increase in risk was small because atrial fibrillation is relatively uncommon in younger adults. The study also could not distinguish between different types or doses of fish oil, so it is unclear whether cod liver oil specifically carries the same risk as high-dose EPA supplements. The mechanism behind this association is still being investigated. If you have existing heart rhythm problems or a family history of atrial fibrillation, though, the finding is worth discussing with a cardiologist before starting or continuing any fish oil supplement.

Effects on the Immune System

Omega-3 fatty acids have well-documented anti-inflammatory properties, which is largely a good thing. But immune suppression is the other side of that coin. Research in healthy volunteers found that even low doses of fish oil reduced certain measures of immune cell activity, including the response of lymphocytes to stimulation and levels of some serum immunoglobulins.

16PubMed Central. Immunosuppressive effects of fish oil in normal human volunteers: correlation with the in vitro effects of eicosapentanoic acid on human lymphocytes

For most healthy people, a mild downward shift in inflammatory immune activity is probably beneficial. But if you are taking immunosuppressant drugs, say after an organ transplant or for autoimmune disease, the additive effect is something your doctor should know about. The same applies during periods when your immune system is already compromised, such as during chemotherapy.

Manufacturing Variability and Mislabeled Products

Dietary supplements, including cod liver oil, are not regulated as rigorously as pharmaceutical drugs in most countries. This creates room for manufacturing errors that can have outsized consequences when fat-soluble vitamins are involved. A striking illustration involved seven young children hospitalized with dangerously high calcium levels after their parents gave them a locally produced fish oil. Analysis revealed the product contained roughly 4,000 times the labeled concentration of vitamin D3, with estimated daily intakes reaching between 266,000 and 800,000 IU.

17PubMed. Vitamin D intoxication due to an erroneously manufactured dietary supplement in seven children

That case involved gross manufacturing negligence, not a problem inherent to cod liver oil itself. But it illustrates why buying from established brands with third-party testing matters. Reputable manufacturers test each batch for vitamin A, vitamin D, and contaminant levels. Products that carry a USP Verified mark or an NSF certification have been independently checked for potency accuracy and purity.

Historical Contamination and How Far We Have Come

People sometimes point to older studies on cod liver oil contaminants as evidence that the product is unsafe. It helps to understand how dramatically the landscape has changed. Baltic cod liver oils collected between 1971 and 1989 contained total PCB concentrations ranging from about 6.7 to 17 micrograms per gram, with toxic equivalency values high enough to raise real concern about chronic exposure, especially in children.

18Elsevier. Most toxic and highly bioaccumulative PCB congeners in cod-liver oil of Baltic origin processed in Poland during the 1970s and 1980s, their TEQ-values and possible intake

Modern refining processes have brought those numbers down dramatically. Activated carbon treatment, molecular distillation, and supercritical fluid extraction were not standard practice decades ago but are now routine for major supplement manufacturers. The gap between a cod liver oil bottle from 1975 and one from today is enormous. That said, production quality is not uniform worldwide, and products sourced from heavily polluted waters or processed by small operations without advanced purification remain a risk in some regional markets.

Who Should Be Especially Careful

Cod liver oil is not inherently dangerous, but certain groups should approach it with more caution than others.

  • Pregnant women: The vitamin A content poses a teratogenic risk during early pregnancy. Standard omega-3 supplements without added retinol are a safer alternative for getting EPA and DHA.
  • People on blood thinners: The anticoagulant effect of omega-3s can compound with medications like warfarin, increasing the risk of bleeding or bruising.
  • Anyone already taking a multivitamin with vitamin A: Stacking two sources of preformed retinol is the most common path to inadvertently exceeding the upper intake level.
  • Children: Upper limits for vitamin A are substantially lower for small bodies. A dose that is safe for an adult could push a child above the ceiling. The EFSA sets the upper limit for infants at just 600 micrograms RE per day, rising to 2,600 micrograms RE for older teenagers.
  • People with liver disease: Because vitamin A is stored in and processed by the liver, pre-existing liver damage raises the risk of toxicity at lower doses.

For healthy adults without these risk factors, a standard dose of cod liver oil, typically one teaspoon or one to two capsules daily, falls well within safe limits for vitamin A, vitamin D, and omega-3 intake. The most practical safeguard is simply reading the label and cross-checking it against your other supplements.

Cod Liver Oil Versus Standard Fish Oil

The question behind many safety concerns is really whether cod liver oil offers enough over standard fish oil to justify the added risks. Standard fish oil supplements derived from the body (not the liver) of oily fish provide EPA and DHA without significant vitamin A or D. If your primary goal is omega-3 intake for cardiovascular or joint health, a regular fish oil capsule gives you that with fewer variables to manage.

Cod liver oil makes the most sense when you also want the vitamin D and A it provides, particularly during winter months in northern latitudes where sunlight-driven vitamin D synthesis drops off. In that context, it functions as a three-in-one supplement. The trade-off is that you need to track your total vitamin A intake more carefully, and you accept a slightly more complex risk profile. Neither product is categorically better; the right choice depends on what else you eat and supplement with.