Commercial fish oil supplements often contain lower concentrations of EPA and DHA than stated on their labels, display lipid oxidation levels above established safety thresholds, and are contaminated with toxic compounds such as heavy metals.

Study: Fish oil supplements in New Zealand are highly oxidised and do not meet label content of n-3 PUFA

Most fish oil pills sold in New Zealand don’t have as much healthy omega-3s as they say, and many are already spoiled or rancid by the time you buy them.

Study: Systematically Investigating the Qualities of Commercial Encapsulated and Industrial-Grade Bulk Fish Oils in the Chinese Market

This study found that many fish oil pills in China don’t have as much EPA and DHA as they say, smell bad because they’re going rancid, and sometimes contain harmful metals — exactly what the claim says.

Study: The Quantitation of EPA and DHA in Fish Oil Dietary Supplements Sold in the United States

This study found that many fish oil pills don't have the amount of healthy fats they say they do on the label. That means you might not be getting what you paid for, but it didn't check for bad stuff like mercury or spoiled oil.

Study: Determination of eicosapentaenoic acid and docosahexaenoic acid contents and the oxidation level of fish oil supplements from Bahrain market

This study checked fish oil pills sold in Bahrain and found that most didn't have as much healthy omega-3s as they claimed, and many were already spoiled from exposure to air — just like the claim says.