The contaminants of most concern are mercury and PCBs. Some brands contain distilled esters, which have a higher concentration of omega-3s and usually a lower concentration of contaminants. Other brands contain the natural triglycerides, which may, according to some evidence, provide more health benefits, but they must be tested thoroughly for contaminants and the omega-3 content will naturally be lower. They may cause or contribute to birth defects and cancers, as well as causing outright symptoms of poisoning. Some, such as swordfish, should be avoided entirely.

If a company uses natural triglycerides, the consumer should check for a "Certificate of Analysis" or COA from an independent laboratory. It is primarily because of mercury and PCBs that people, particularly women and children, are advised to limit their consumption of certain species of fish. Most sources suggest that the correct daily dosage of fish oil is 2-3 capsules, but that is a very non-specific suggestion. What is the Best Dosage of Fish Oil. A capsule usually contains 500-1000mg of oils. The content of the capsules on the market varies greatly.

This document will verify that there was no detectable level of contaminants.