In a study of fish oil products in China, most commercial capsules met quality standards for oxidation, but nearly all bulk industrial samples did not, showing that bulk fish oil is more likely to be degraded.

A sample of 36 fish oil supplements sold in New Zealand was tested for freshness, and most showed signs of oxidation, meaning they had degraded beyond accepted quality standards.

A study found that more than half of fish oil supplements sold in Bahrain had signs of spoilage beyond safe limits, which may reduce their health benefits and increase exposure to potentially harmful chemicals formed during oxidation.

Testing of fish oil supplements in Bahrain found that more than half had high levels of early-stage oxidation markers, but few showed signs of advanced spoilage; however, about one in three still exceeded safety limits for total oxidation.

A study of 14 fish oil supplements sold in Bahrain found that most exceeded safety limits for oxidation, and only a small fraction contained the amount of EPA and DHA listed on their labels.

In fish oil products sold in China, measurements of chemical oxidation do not reliably predict whether the oil smells fishy or rancid to consumers.