Cod liver oil isn’t bad for you at the right dose, but it’s easier to overdo than most people realize. The main concern is vitamin A: one tablespoon of cod liver oil contains roughly 4,080 micrograms of preformed vitamin A, which already exceeds the safe upper limit of 3,000 micrograms per day set for adults. That single detail is what separates cod liver oil from regular fish oil and makes dosing matter more.
The Vitamin A Problem
Regular fish oil gives you omega-3 fats without significant amounts of vitamins. Cod liver oil, because it comes from the liver of the fish, is loaded with preformed vitamin A (retinol) and vitamin D. That tablespoon delivering over 4,000 micrograms of retinol is a problem because the tolerable upper intake for adults is 3,000 micrograms per day. This means a full tablespoon already puts you above the ceiling before counting any vitamin A from food, multivitamins, or fortified products you consume the same day.
A teaspoon (roughly 5 ml, or one-third of a tablespoon) brings the vitamin A content down to a range most people can handle safely. Many capsule-form products are formulated at this lower dose. But if you’re taking a tablespoon because that’s what the bottle says, or because your grandmother did, you could be accumulating more retinol than your body can process.
What Happens if You Take Too Much
Vitamin A toxicity doesn’t usually happen from one large dose (though that’s possible at extreme amounts). The more realistic scenario with cod liver oil is chronic toxicity, which develops from regularly exceeding about 8,000 micrograms per day over weeks or months. But harmful effects on bone have been observed at lower thresholds. The UK Expert Committee on Vitamins and Minerals considers anything above 1,500 micrograms per day “inappropriate” based on potential bone effects.
Early signs of chronic excess include dry and cracking skin, hair loss, brittle nails, fatigue, and loss of appetite. More serious consequences affect your bones and liver. Prolonged overconsumption accelerates bone breakdown and reduces new bone formation, leading to osteoporosis, bone pain, and increased fracture risk. Your liver, which stores and processes retinol, can develop fatty changes, scarring, and eventually cirrhosis. Headaches, nausea, and blurred vision are also common signals that something is off.
Pregnancy Is a Special Case
Pregnant women and women trying to conceive face an additional, serious risk. Excess preformed vitamin A during pregnancy can cause birth defects including spinal abnormalities, malformations of the ears, eyes, heart, kidneys, and limbs, and cleft palate. The UK government explicitly recommends that pregnant women and those planning pregnancy avoid cod liver oil and other supplements containing vitamin A unless directed by a doctor. The European Food Safety Authority caps intake at 3,000 micrograms per day for women of childbearing age, but given how easy it is to exceed that with cod liver oil, the practical advice is to skip it entirely during this period and get omega-3s from regular fish oil instead.
The Benefits Are Real
None of this means cod liver oil is purely harmful. At a sensible dose, it delivers a useful combination of omega-3 fatty acids, vitamin D, and vitamin A in one supplement. That vitamin D content (about 34 micrograms, or 1,360 IU per tablespoon) is a meaningful amount, especially for people living in northern climates or spending limited time outdoors.
The omega-3 fats in cod liver oil, primarily EPA and DHA, have solid evidence behind them for heart health. A large 2019 review of clinical trials concluded that omega-3 supplementation reduces the risk of heart attack, coronary heart disease death, and overall cardiovascular disease in a dose-related way. Individual trials have found reductions in heart attack rates of 28%, cardiovascular death by 19 to 20%, and stroke risk by 20 to 28% in certain populations. The benefits appear strongest in people who don’t eat much fish and those with existing heart disease.
That said, a typical cod liver oil serving provides less EPA and DHA than a dedicated high-strength fish oil supplement. If your primary goal is omega-3 intake, regular fish oil may actually deliver more of the fatty acids you’re after without the vitamin A ceiling to worry about.
Contaminants and Freshness
Because cod liver oil comes from a specific organ that filters toxins, people reasonably worry about mercury, PCBs, and other pollutants. Testing of commercial products has found that mercury levels in fish oil capsules are generally very low, and the most toxic form of mercury (methylmercury) is typically below detection limits. Some PCB residues and traces of older pesticides like DDT have been detected in certain products, particularly canned cod liver, but levels in refined supplement-grade oils are far lower. Reputable brands test for contaminants and publish results, so checking for third-party verification is worth the few seconds it takes.
Rancidity is a subtler concern. Omega-3 fats are chemically fragile and oxidize easily, producing compounds that may cancel out benefits or cause low-level harm. Animal studies show oxidized fats can damage tissues, and while one short human trial found no acute toxicity markers from oxidized fish oil over seven weeks, it didn’t assess longer-term risks like arterial damage or DNA effects. If your cod liver oil smells strongly fishy or bitter rather than mildly oceanic, it’s likely gone off. Store it in the refrigerator, use it before the expiration date, and don’t buy more than you’ll finish in a few months.
Blood Thinners and Medications
Omega-3 fats have a mild blood-thinning effect, which raises questions about interactions with anticoagulant medications like warfarin. In practice, the risk appears smaller than once thought. A retrospective study of patients on warfarin for atrial fibrillation or blood clots found that concurrent fish oil use did not significantly affect clotting times or bleeding rates. A randomized trial of 610 patients taking either aspirin or warfarin found no significant increase in bleeding when 4 grams of omega-3s were added daily. Isolated case reports of elevated clotting times exist, but confounding factors made it hard to pin the cause on fish oil alone. Still, if you take blood-thinning medication, letting your doctor know you’re using cod liver oil is a reasonable precaution.
How To Use It Safely
The simplest way to avoid problems is to stick to a teaspoon or less per day if you’re using liquid cod liver oil, or follow the capsule count on a reputable brand’s label. At that dose, most adults will stay under the vitamin A upper limit while getting useful amounts of vitamin D and omega-3s. Factor in other sources of preformed vitamin A in your diet: liver, fortified milk, eggs, and any multivitamin you take. If those sources already contribute significant retinol, cod liver oil may push you past a comfortable margin.
If you’re pregnant, planning to become pregnant, or have liver disease, regular fish oil is a safer choice for omega-3 supplementation. For everyone else, cod liver oil at an appropriate dose is a well-tolerated supplement with genuine nutritional value. The problems start when people treat it casually and take more than they need.