Omega-3 fatty acids in the form of fish oil have positive benefits; these include lowering the risk of cardiovascular disease and lowering triglyceride levels, according to MayoClinic.com. As with any supplement, however, omega-3 fish oils can also have adverse effects. While negative effects don't occur often, they can occur, especially at high doses. Some effects may have serious consequences.
Bleeding
Omega-3 fish oil supplements may increase the risk of bleeding at high doses, although little evidence suggests that this occurs often or that it might occur at lower doses, MayoClinic.com states. Bleeding may occur due to prolonged bleeding time, an increase in breakdown of blood clots and a decrease in von Willebrand factor, one of the proteins that help blood to clot. Omega-3 fish oils may also decrease the tendency of platelets to stick together, which helps plug cuts and other breaks in blood vessels. Hemorrhagic stroke can occur, causing severe headache, sudden vision loss, weakness and numbness on one side of the body. Nosebleeds and blood in the urine, along with bright red blood on stools or black and tarry stools may indicate increased bleeding, eMedTV warns.
Stomach Upset
People taking omega-3 fish oil supplements may experience unpleasant stomach upset, with nausea, burping, heartburn, fishy aftertaste and bloating. Severe diarrhea may occur at high doses, MayoClinic.com warns.
Increased LDL Cholesterol Levels
Omega-3 fish oil may raise low-density lipoprotein, or LDL, cholesterol levels by 5 to 10 percent, MayoClinic.com reports. This LDL is the "bad" cholesterol, and raising LDL levels may result in increased risk of atherosclerosis and heart disease.
Heavy Metal Contamination
Fish oil supplements may contain excessive amounts of heavy metals and toxins, such as methylmercury, dioxins and polychlorinated biphenyls -- more commonly known as PCBs. Toxins normally accumulate more in the fish meats than in the oils, MayoClinic.com explains, making ingestion from fish oils unlikely but possible. Children and pregnant and nursing women are at the highest risk from ingesting toxins. The American Family Physician reported in 2004 that high-quality supplements rarely contain any contaminants.
Blood Pressure Changes
Omega-3 fish oils lower blood pressure slightly, by between 2 to 5 mmHg, according to MayoClinic.com. This may be an undesirable side effect in someone who already has low blood pressure.
Increased Blood Sugar Levels
Diabetics may experience increased fasting blood sugar levels while taking omega-3 fish oils, although increases are slight and appear to have no significant effects.
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