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 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.

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.

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.

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.

Fish oil supplements that have degraded due to oxidation tend to have less EPA and DHA than what is listed on the label, which may explain why some products contain lower amounts than claimed, though other factors can also cause this discrepancy.