While those with heart disease and diabetes have often been reported to benefit from supplementation with fish oil,^{12, 13} both groups should check with their doctor before taking more than 3 grams of fish oil per day for several months. Elevations in blood sugar and cholesterol levels may occur in some people who take fish oil.¹⁴

The increase in blood sugar appears to be related in part to the amount of fish oil used.¹⁵ The impairment of sugar metabolism sometimes caused by supplementation with fish oil has been prevented by the addition of half an hour of moderate exercise three times a week.¹⁶

While supplementation with fish oil consistently lowers triglycerides, the effect of fish oil on LDL (“bad”) cholesterol varies, and in some people, fish oil supplementation has been reported to increase LDL levels.¹⁷

There has been one case report in which a woman with a known allergy to crabs developed a severe allergic reaction after ingesting fish oil.¹⁸