While those with heart disease and diabetes often benefit from fish oil (the primary source of DHA in the diet),^{16, 17} such people should check with their doctor before taking more than 3 or 4 grams of fish oil per day for several months. Elevations in blood sugar have sometimes been reported,¹⁸ though this may simply be due to small increases in weight resulting from high dietary fish oil.¹⁹ While DHA combined with EPA from fish oil consistently lowers triglycerides, it occasionally increases LDL cholesterol.²⁰

According to a report in a Japanese medical journal, three people at high risk for colon cancer developed a variety of cancers after one to two years of supplementation with DHA.²¹ To date, this report has not been confirmed by other researchers. To the contrary, test tube studies report that DHA is toxic to cancer cells²² and may someday be considered as an adjunct to conventional treatment for cancer.²³ Similarly, animal studies suggest that DHA may inhibit cancer.²⁴