A fresh fish oil should test under 5 meq/kg peroxide value. In a survey of 171 omega-3 supplements sold in Canada, half failed at least one of the three industry freshness limits, and another 18% were within one to three years of failing. Your cat does not need a lab report; four storage habits and one label check cover most of the risk. What follows is how the limits are set, which container survives, and how much oil a 4 kg cat actually needs per day.
Three numbers decide whether the oil is still good
Peroxide value (PV) measures primary oxidation: the peroxides formed when oxygen attacks EPA and DHA. The p-anisidine value (p-AV) measures the secondary products, aldehydes and ketones, which is where the fishy smell and taste come from. TOTOX combines both: TOTOX = 2 x PV + p-AV. A product can pass on peroxide and still fail on TOTOX, which is why careful sellers publish all three.
The fresh line and the discard line are not the same line
The Global Organization for EPA and DHA (GOED) sets voluntary ceilings of PV 5 meq/kg, p-AV 20 and TOTOX 26. Pharmacopoeial monographs are looser: the European Pharmacopoeia allows PV 10 and p-AV 30 for fish oil rich in omega-3 acids. For a cat supplement, treat GOED as fresh and the pharmacopoeia numbers as the point where the oil is no longer worth giving.
| Measure | Fresh (GOED) | Pharmacopoeia ceiling | Discard |
|---|---|---|---|
| Peroxide value | 5 meq/kg or less | 10 meq/kg | above 10 meq/kg |
| p-Anisidine value | 20 or less | 30 | above 30 |
| TOTOX | 26 or less | about 50 | above 30, or any fishy smell |
In a 47-product survey of the New Zealand market, 72% met the GOED peroxide limit, 86% met p-AV and 77% met TOTOX, yet 98% met the looser peroxide limit of 10. Most commercial oil is not harmful; a minority is stale, and stale oil is not something a bigger dose can fix.
The bottle is the weak point, not the brand
A 49-day shelf test published in Food Science and Nutrition in 2023 compared capsules, chewable gels and syrups in pump bottles. Capsules barely moved. Two of six syrups crossed the TOTOX limit of 26 by day 7, and one reached a peroxide value of 44.6 meq/kg by day 49, close to nine times the GOED ceiling. The reason is headspace: every pump stroke pulls air in, and the oil film left on the nozzle keeps oxidising between doses.
| Form | Peak PV (meq/kg) | Peak TOTOX | Days to exceed TOTOX 26 |
|---|---|---|---|
| Capsule or softgel | 7.6 | 30.4 | 29 in one sample, never in others |
| Chewable gel | 26.1 | 65.8 | 0 to 14 |
| Syrup in a pump bottle | 44.6 | 96.9 | 7 |
In a cat the gap gets wider because the daily dose is small. A 4 kg cat at 30 mg/kg needs 120 mg of EPA plus DHA a day. A 250 mL bottle of oil at 200 mg per mL holds 50,000 mg, which is more than 400 days of that dose. An open bottle of polyunsaturated oil that lasts over a year is a storage problem no brand can solve.
Storage rules that buy weeks
- Liquid oil: refrigerate after opening and finish it within 90 days, which is the interval most manufacturers print; write the open date on the cap.
- Capsules: keep them dark, dry and at room temperature until the expiry date; the fridge helps only if your kitchen sits above 25 C.
- Heat: oxidation roughly doubles for every 10 C of warmth, so the cupboard above the stove or a sunny sill is worse than the fridge door.
- Container: keep the original dark glass or opaque bottle; decanting into a wide-mouth jar doubles the surface exposed to air.
- Freezer: unnecessary for oil and it makes capsules brittle, so skip it.
Smell is a late alarm, not an early one
Peroxides have almost no odour. The fishy, paint-like note comes from secondary aldehydes that appear after the peroxide value has already climbed, so a bottle that smells has usually been failing for weeks. Cats are more sensitive than we are: a cat that eats a fish-flavoured meal happily but walks away from the same food once the oil is stirred in is telling you about the oil, not the food.
How much EPA and DHA a cat actually needs
The 2006 NRC recommended allowance for adult cats is 0.025 g of EPA plus DHA per 1,000 kcal of diet, about 6 mg a day for a cat eating 250 kcal. That is a nutritional floor, not a therapeutic dose. Supplement ranges used in practice run 20 to 40 mg/kg of combined EPA and DHA per day, which for a 4 kg cat is 80 to 160 mg. Read the back of the label: a 1,000 mg capsule of 30% concentrate holds 300 mg, so a 4 kg cat gets roughly 0.4 of a capsule. The dose maths is here, and fish oil versus algal oil is compared here.
A 30-second label check
- Milligrams of EPA and DHA per capsule, not milligrams of fish oil.
- Concentration: 30% is standard, 60 to 80% is a concentrate, so adjust the number of capsules rather than the target dose.
- An antioxidant on the list, usually mixed tocopherols or rosemary extract. In the 171-product survey 89% contained one and half still failed a freshness limit.
- A batch or harvest date, plus third-party oxidation data such as an IFOS report or a published certificate of analysis.
- Packaging: dark glass, nitrogen flushed, and a bottle small enough to finish in two months.
When stale oil turns into a vet visit
Signs blamed on oxidised oil in cats are mostly digestive: vomiting within a few hours, soft stool, or a sudden refusal to eat. Stop the supplement, keep the bottle, and call your vet if vomiting repeats more than twice in 24 hours or the cat goes a day without eating. Peroxide load and fat-soluble vitamins matter more in cats with kidney disease or pancreatitis, where the diet is already restricted.
This article is not a substitute for veterinary diagnosis. The dose, the product and any decision to stop a supplement should be confirmed with your veterinarian, who knows the cat's bloodwork and other medications.
The short version
Buy capsules when you can, refrigerate liquid oil, write the open date, finish liquid within 90 days, and discard it at a peroxide value above 10, a p-anisidine value above 30, or a TOTOX above 30 with any fishy smell. At 120 mg a day a 250 mL bottle is a 14-month supply for one cat, which is already too long for an open bottle of PUFA oil.
Frequently asked
How can I tell if my cat's fish oil has gone rancid without a lab test?
You cannot measure peroxide value at home, so use three proxies. Check the open date against 90 days for liquid oil, keep the bottle in the fridge, and smell it: a sharp fishy or paint-like note means secondary aldehydes are present, which appears after peroxide has already climbed. If the cat suddenly refuses food that contains the oil, treat the oil as suspect and replace it with capsules.
Are capsules really more stable than liquid fish oil?
In a 49-day shelf test, capsules stayed at or below 7.6 meq/kg peroxide while syrups in pump bottles reached 44.6 meq/kg and a TOTOX of 96.9, with two of six syrups crossing the TOTOX limit of 26 by day 7. The difference is air contact: a sealed softgel has almost no headspace, while every pump stroke pushes air into the bottle.
What daily dose of EPA and DHA does a 4 kg cat need?
The NRC recommended allowance is 0.025 g per 1,000 kcal, roughly 6 mg a day for a cat eating 250 kcal, but supplement ranges in practice are 20 to 40 mg/kg of combined EPA and DHA, or 80 to 160 mg for a 4 kg cat. A 1,000 mg capsule of 30% concentrate supplies 300 mg, so the dose is about 0.4 of a capsule; confirm it with your vet.