Cod liver oil is extracted from the liver of cod fish. As a source of healthy omega-3 fatty acids and other vitamins, cod liver oil is a beneficial nutritional supplement. However, as with most supplements, it also has some potential side effects.
The omega-3 fatty acids in cod liver oil benefit numerous bodily functions, such as reducing triglyceride levels (one factor in total cholesterol) and aiding in general cardiovascular health. Other potential benefits of omega 3s include reducing cancer risk, aiding in cognitive function and assisting in weight loss. Cod liver oil is also a major source of vitamin A, essential for a healthy immune system and bone growth, and vitamin D, which helps build and maintain strong bones.
Side Effects: Omega-3s
Although low-to-moderate doses are usually safe for healthy adults, cod liver oil may cause minor gastrointestinal upset. The National Institutes of Health (NIH) indicates that excessive doses of omega-3 fatty acids may increase the risk of bleeding---from nosebleeds or blood in the urine to a stroke (hemorrhagic bleeding). Fish oil may also "interact with certain medications, including blood thinners and drugs used for high blood pressure."
Side Effects: Vitamins
The NIH also warns against taking fish oil supplements in higher amounts and/or for prolonged periods of time because "fish liver oil products (such as cod liver oil) may increase the risk of vitamin A or D toxicity." The NIH's Office of Dietary Supplements reports that high storage level of vitamin A can result in birth defects, liver abnormalities, reduced bone mineral density that may result in osteoporosis, and central nervous system disorders. Excessive vitamin D can result in symptoms ranging from nausea, weakness and weight loss to hypercalcemia, which can lead to cognitive dysfunction, kidney stones or failure. Individuals taking multivitamins regularly or in high doses should inform their physician.
Up to 3g per day of omega-3 fatty acids is generally regarded as safe by the Food and Drug Administration. However, if you are diabetic, pregnant, have high blood pressure or cholesterol, or are sensitive or allergic to fish or foods with omega-3 fatty acids such as nuts, consult with your doctor about taking cod liver oil.
People concerned about the mercury levels in fish need not worry about mercury levels in cod liver oil supplements. Supplements contain almost no mercury, compared to fresh fish such as shark, swordfish or tilefish.