Adverse Effects
Per tablespoon (13.6g), cod liver oil contains 136% of the established Tolerable Upper Intake Level (UL) for Preformed Vitamin A (Retinol). Vitamin A accumulates in body fat, and can reach harmful levels sufficient to cause hypervitaminosis A. Pregnant women may want to consider consulting a doctor when taking cod liver oil because of the high amount of natural forms of vitamin A such as retinol. High doses of synthetic vitamin A (retinoids) have been shown to cause birth defects.
The risks of hypervitaminosis and of exposure to environmental toxins such as mercury, polychlorinated biphenyls (PCBs), dioxins, and other contaminants, are reduced when purification processes are applied to produce refined fish-oil products, which consequently contain raised levels of omega-3 fatty acids, such as EPA and DHA.
A high intake of cod liver oil by pregnant women is associated with a nearly fivefold increased risk of gestational hypertension, although this study did not control for mercury, which can be present in harmful amounts in fish and which is another cause of hypertension.
Some urge caution when taking cod liver oil and other fish-based supplements since they may contain elevated levels of toxins such as mercury and PCBs found in fish. Some supplement companies regularly test cod liver oil for purity. Consumerlab.com tested 44 fish and cod liver oil supplements and found that all were free of detectable levels of mercury.
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