Fish oil is among the most widely sold supplements in the United States and among the most chemically fragile. The property that defines it is also what makes it unstable.
Why the molecule is vulnerable
Omega-3 fatty acids are polyunsaturated, meaning they carry multiple carbon-carbon double bonds along the chain. Those bonds are the structural feature that gives the molecules their properties.
They are also chemically reactive. The hydrogen atoms adjacent to a double bond are loosely held and readily abstracted by oxygen, which starts a chain reaction.
Once initiated, oxidation propagates: each reacted molecule generates a species capable of attacking another. This is why deterioration accelerates rather than proceeding at a constant rate.
What oxidation produces
The first products are peroxides, which are unstable and break down further into aldehydes, ketones and short-chain compounds.
These secondary products are what produce the characteristic rancid smell of old fish oil, detectable long before any laboratory analysis would be commissioned.
Industry measures track both stages — a peroxide value for the early products and an anisidine value for the later ones — because either alone gives an incomplete picture.
Why the capsule matters
A softgel is a delivery format and a barrier. It reduces oxygen exposure during storage, which is genuinely useful, and it removes the taste and smell from the consumer's experience.
Removing the smell also removes the detection mechanism. A liquid oil announces its condition; an encapsulated one does not until the shell is broken.
Splitting a capsule and smelling the contents is the simplest available check, and it is one most purchasers never think to perform.
What accelerates the process
Heat, light and oxygen all speed oxidation, which is why storage conditions between the factory and the medicine cabinet affect what is eventually swallowed.
A bottle left in a warm car, stored above a stove or kept in a bathroom with daily humidity swings deteriorates faster than the shelf life implies.
Antioxidants such as tocopherols are commonly added to slow initiation, and refrigeration after opening reduces the rate further.
Why this complicates the research
Trials of omega-3 supplementation have produced inconsistent results across a large literature, and oxidation state is one proposed source of that inconsistency.
If products used in different studies differed in oxidative quality, they were not delivering the same intervention despite carrying the same label figures.
Whether omega-3 supplementation is appropriate for any individual is a clinical question, particularly alongside blood-thinning medication, and belongs with a physician or pharmacist.