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.

We analyzed the available evidence and found that fish oil supplements that have oxidized—meaning they’ve broken down due to exposure to air, light, or heat—tend to contain less EPA and DHA than what’s listed on the label [1]. This doesn’t mean all under-labeled products are due to oxidation, but when oxidation occurs, it appears to reduce the actual amount of these omega-3 fatty acids in the capsule. We saw no studies that contradicted this pattern. The evidence we’ve reviewed leans toward oxidation being one factor that can lead to lower-than-claimed EPA and DHA levels, though other issues like manufacturing errors or poor storage may also play a role. What we’ve found so far suggests that if a fish oil supplement smells fishy, tastes rancid, or has passed its expiration date, the omega-3 content may be lower than advertised—not because the label is intentionally wrong, but because the oil has degraded over time. This doesn’t prove all supplements are unreliable, but it does mean freshness matters. To get closer to the labeled amount, look for products with dark bottles, refrigerated storage, and recent expiration dates.