What Is Cod Liver Oil? A Pet Owner's Guide to EPA, DHA, and Vitamins A & D

Cod liver oil has been a household name for generations, mostly because generations of parents made their kids take a spoonful of it. What’s less well known is exactly what’s in it, and why it’s become a common ingredient in pet supplements too. Here’s a straightforward look at what cod liver oil actually is.

Where it comes from

Cod liver oil is extracted from the liver of the Atlantic or Arctic cod. Unlike standard fish oil, which is typically pressed from the flesh, liver oil is drawn from the organ where the fish stores its fat-soluble nutrients. That’s the source of cod liver oil’s distinct nutritional profile — it’s not just an omega-3 oil, it’s an omega-3-plus-vitamin oil.

The four nutrients that matter most

EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid) are the two long-chain omega-3 fatty acids found in cod liver oil. Both are essential fatty acids, meaning a dog or cat’s body can’t produce them on its own — they have to come from diet. EPA and DHA are widely studied for their role in normal heart, brain, and cell membrane function across mammals.

Vitamin A supports vision, immune function, and skin health, and occurs naturally and abundantly in cod liver oil.

Vitamin D plays a role in calcium regulation and bone health, and is one of the few vitamins that’s genuinely difficult to get from a typical diet without a fortified source — cod liver oil is one of the most naturally concentrated dietary sources available.

How it differs from plant-based omega-3s

Some pet supplements use flaxseed oil or algae oil as an omega-3 source instead of fish oil. The distinction matters: flaxseed oil contains ALA (alpha-linolenic acid), a shorter-chain omega-3 that the body has to convert into EPA and DHA to use — a conversion that’s notably inefficient in dogs and cats. Algae oil can supply DHA directly but typically lacks EPA and doesn’t carry the vitamin A and D content that liver oil does. Cod liver oil delivers EPA, DHA, vitamin A, and vitamin D all from a single, naturally occurring source.

Why the source matters

Not all cod liver oil is equal. Oil from wild-caught Arctic cod, cold-pressed at low temperatures, retains more of its naturally occurring nutrient profile than oil that’s heavily refined or sourced from farmed fish. That’s the standard we hold our own formula to — you can read more about where our cod liver oil comes from on our About page.

If you’re weighing a supplement for your dog or cat, the honest answer is that a plain “fish oil” label doesn’t tell you much. Knowing whether it’s liver oil, and whether it’s wild-caught, tells you almost everything.