Fish oil which is rancid or oxidized may have an unpleasant odor and taste, although it would be hard to tell if you are taking a softgel. Oxidized fish oil may also raise levels of LDL "bad" cholesterol, as explained in the "Quality Concerns" section of our Fish Oil and Omega-3 Fatty Acid Supplements Review
To find a fish oil supplement that has been shown to be fresh and not rancid, see the Results table
in our fish oil review and look for products that received a checkmark in the "Freshness" column.
Some studies have suggested that oxidation of fish oil in supplements is common. These studies have generally not taken into account the fact that the citrus flavoring added to fish oil supplements can interfere with oxidation tests and falsely indicate that an oil is oxidized. ConsumerLab factors this possibility into its reporting to avoid mischaracterizing the quality of a product.
Be sure to also see our tips for storing and keeping your fish oil supplement fresh