Omega-3 Oxidation, Mercury and Reading the Label

Three practical worries come up when people shop for omega-3: whether the oil is fresh, whether mercury is a problem, and what the label really says. This guide sets out what the sources say about each. It is general information for healthy adults, not medical advice. For the full set of buying checks, see our buying guide.

Why omega-3 oils go rancid

The Linus Pauling Institute (LPI) explains that polyunsaturated fats turn rancid (oxidize) more easily than saturated fats, and that vitamin E, a fat-soluble antioxidant, helps protect them. Two numbers describe the damage: the peroxide value (PV) for early oxidation and the anisidine value (AV) for later products, combined as TOTOX, which is 2 times PV plus AV. GOED, the Global Organization for EPA and DHA Omega-3s, is a trade association. Its voluntary monograph, in the January 2022 version we read (a newer version dated July 2026 appears to exist, but we could not open it), sets maximums of 5 meq/kg for PV, 20 for AV and 26 for TOTOX, and says the AV and TOTOX limits do not apply to flavored oils.

What a 2015 study found

Jackowski and colleagues analyzed 171 omega-3 supplements from 49 brands bought in Ontario, Canada, in 2014. Half of the products with usable results exceeded at least one of the voluntary limits: 17% had a peroxide value above 5, 41% an anisidine value above 20 and 39% a TOTOX above 26. Bulk oils and children's products had higher TOTOX values than softgels, and 89% of the products contained an antioxidant additive such as vitamin E or rosemary extract, so an antioxidant is not a guarantee. The authors wrote that no North American agency regulated oxidation levels in all omega-3 supplements. Most of them work for Pivotal Therapeutics, a Canadian company whose own omega-3 product was among those analyzed, and the products were bought in Canada, so US shelves may differ.

A commentary from the University of Auckland on that study and similar ones, whose authors declared no conflicts of interest, stressed that the biological effects and health consequences of consuming oxidized fish oil supplements are not yet established. It quoted the European Food Safety Authority's 2010 opinion that information on oxidation levels and related toxic effects in humans is lacking. So rancid oil is a quality problem with an unknown health impact, and we do not claim more than that.

Practical freshness checks

• Smell and taste. The commentary links secondary oxidation products to rancid smells and off flavors. A rancid smell is a reason to stop and contact the seller. NCCIH and LPI both list an unpleasant or fishy aftertaste among common side effects, so an aftertaste alone is not proof.
• Dates and storage. The 2015 study noted that oxidation can keep rising while a product is still within its best-before date. Check the date, keep the container closed and follow the label's storage line.
• Format. In that study, bulk oils scored worse than unflavored softgels, though one study cannot rank brands.
• Outside testing. FDA does not test dietary supplements before they are sold, and the firm making or selling one is responsible for its safety and labeling, which is why voluntary programs exist. USP says its Verified Mark means the listed ingredients are present in the declared amounts and that harmful levels of specified contaminants are absent. NSF certification includes a label-claim review and a contaminant review. IFOS, run by SGS Nutrasource, is specific to fish oil and says it posts results by lot number. Those are the programs' own descriptions. Searches for IFOS certified fish oil on Amazon (opens in a new tab), USP Verified fish oil on Amazon (opens in a new tab) and NSF certified omega-3 on Amazon (opens in a new tab) match words only, so confirm the mark on the pack and in the program's own listing.

Mercury: two different questions

Mercury in fish. FDA and EPA advice about eating fish, revised in October 2021, sorts fish into Best Choices, Good Choices and Choices to Avoid by mercury level. For pregnancy and breastfeeding, a serving is 4 ounces and the advice is 2 to 3 servings a week from Best Choices or 1 serving from Good Choices. The Choices to Avoid, with the highest mercury, are king mackerel, marlin, orange roughy, shark, swordfish, Gulf of Mexico tilefish and bigeye tuna. Salmon, sardines, anchovies, herring and Atlantic mackerel are on the Best Choices list. That advice is about eating fish, not about capsules.

Mercury in supplements. LPI reports that several independent US laboratory analyses found commercially available omega-3 supplements free of methylmercury, PCBs and dioxins, and explains that mercury accumulates in fish muscle rather than in the fat. A 2003 study by Foran and colleagues measured mercury in five over-the-counter fish oil brands and found levels from not detectable, under 6 micrograms per liter, to 10 to 12 micrograms per liter, which the authors called similar to normal blood levels. Five brands in 2003 is a small, old sample. LPI adds that fish liver oils tend to carry more PCBs and other fat-soluble contaminants than fish body oils, and that more highly refined, deodorized oils carry less. GOED's voluntary monograph, in that 2022 version, sets mercury at under 0.1 mg/kg. Contaminant checks are where outside testing earns its keep.

Reading the label

• Serving size. FDA says a serving is the largest amount the label recommends per eating occasion. Directions of "1 to 3 softgels" mean the panel describes 3.
• EPA and DHA lines. The panel must name dietary ingredients and give their quantities. Look for EPA and DHA in milligrams or grams per serving, not just "fish oil" or "omega-3" in total.
• Total fat and calories. Fish oil is fat, so the panel can also list calories and fat. Our guide to reading the Nutrition Facts label explains those lines.
• Source. FDA's allergen law covers dietary supplements and requires the species of fish to be named, but excludes highly refined oils, so read the ingredient list as well as any "Contains" line. Our guide to the nine major allergens explains the law.
• Other Ingredients. Capsule shells, glycerin and added antioxidants appear here in descending order by weight; FDA's own example list for a cod liver oil product reads "Cod liver oil, gelatin, water, and glycerin".
• Lot number and date. Note the lot number, in case the brand publishes results by lot, and the best-before date.
• Disclaimer. A structure/function statement must be followed by FDA's disclaimer. Treat a disease promise as a warning sign, since FDA says a supplement represented as treating or preventing disease is regulated as a drug.

Who should ask first

Talk to a doctor or pharmacist before you start if you are pregnant or breastfeeding, take blood-thinning medicine, have a fish or shellfish allergy or are buying for a child. FDA advises everyone to check with a health care professional before using a supplement, because some interact with medicines. The label on your pack is the authority on dose, ingredients and warnings.