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What are the side effects of fish oil supplements?

Fish oil supplements are dietary supplements that contain oil from cold water fish such as mackerel, salmon, black cod, albacore tuna, sardines, and herring. The active ingredients in fish oil supplements are essential fatty acids known as omega-3 fatty acids. They typically include eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA).

The most commonly reported side effects of fish oil supplements include fishy odor on the breath, upset stomach, and greasy stools. A more serious side effect can be an increase in LDL cholesterol, a harmful form of cholesterol. Large doses of fish oil supplements may also cause weight gain and decrease the absorption of vitamins A, E, D, and K from the stomach. If you have diabetes, use caution when taking fish oil supplements, they are reported to increase blood sugar.

Taking large amounts of fish oils over a long period of time may increase the risk of mild bleeding resulting in nosebleeds and bruising.

It's not a good idea to use cod liver oil as a fish oil supplement because it contains very high levels of vitamin A. High doses of vitamin A can cause headache, dry skin, itching, and liver damage.

Sources:
Fasching, P. "Fish Oil Supplementation Versus Gemfibrozil Treatment in Hyperlipidemic NIDDM. A Randomized Crossover Study," Hormonal and Metabolic Research 28 (1996).
Gray, D.R. "Fish Oil As An Adjuvant in the Treatment of Hypertension," Pharmacotherapy 16 (1996).
Micromedex Healthcare Series. Micromedex, Inc., 2001.

This answer prepared 4/20/01.