Doing real work · Omega-3 oil

Fish oil

Also appears on labels as: salmon oil, salmon oil (source of dha), menhaden fish oil, menhaden fish oil (preserved with mixed tocopherols), pollock oil, herring oil

Oil pressed from oily fish, rich in the long-chain omega-3 fatty acids EPA and DHA.

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Where it comes from: Fish reduction plants, usually stabilized with vitamin E immediately after pressing.
Why it's in the product: One of the few ingredients with solid clinical evidence behind it: EPA and DHA have measurable benefit for canine osteoarthritis, skin conditions, and puppy neurological development.
Safety and toxicity: Safe. Two things to know. Fish oil oxidizes fast, and rancid fish oil is both useless and mildly pro-inflammatory, so the preservative matters. Cod liver oil specifically is high in preformed vitamins A and D, which are the two vitamins where excess is a genuine toxicity risk.
Interactions and who should be careful: High-dose omega-3 can extend bleeding time. Tell your vet before surgery if your pet gets a fish oil supplement on top of the food.

General reference, not veterinary advice. Dose, breed, and existing conditions change the answer, so talk to your vet before changing what your pet eats. Think we got this wrong? Tell us and we log it publicly.