Fish Oil

The marine source of EPA and DHA — and the nutritional intervention that came out strongest in the best canine joint review to date.

Fish Oil for Pets: The Supplement With the Best Evidence Behind It

Fish oil is the marine source of two long-chain omega-3 fatty acids, EPA and DHA. Among everything sold for animal joints, it is the one with the strongest research behind it — and we say that knowing it is not a product we make.

Here is what it does, how much to give a dog, cat or horse, how to read a label that is designed to confuse you, and the storage mistake that quietly wastes most of what people buy.

What Fish Oil Actually Is

Fish oil is pressed from oily fish — anchovy, sardine, menhaden, salmon, mackerel. Its value is not the oil itself but two fatty acids in it: EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid).

These are the long-chain omega-3s. Plant oils supply a different, shorter omega-3 called ALA, and the body has to elongate ALA into EPA and DHA before it can use it that way. That conversion is inefficient in dogs and worse in cats.

This is the honest limit we already state on our own hemp seed oil entry: hemp seed oil is an excellent source of omega-6 and GLA, and it is not a substitute for marine omega-3. Tufts University's Cummings School covers the fatty acid families clearly.

What the Evidence Says

The 2022 systematic review and meta-analysis of enriched diets and nutraceuticals in canine and feline osteoarthritis — 57 articles, 72 trials, weighted by study quality — reported "evident clinical analgesic efficacy for omega-3-enriched diets, omega-3 supplements and cannabidiol (to a lesser degree)." The same review found a "very marked non-effect" for glucosamine and chondroitin. You can read it here.

That ranking is worth sitting with. Of the nutritional options on the shelf, omega-3 came first. See our glucosamine entry for what came last.

What Fish Oil May Support

As part of a complete and balanced diet, EPA and DHA support normal joint comfort and mobility, normal skin barrier function and coat condition, normal heart and kidney function, and — from DHA specifically — normal brain and retinal development in puppies and kittens.

These are nutritional support roles, not treatments. Any animal with a diagnosed condition needs a veterinarian, not a supplement aisle.

How Much Fish Oil to Give

This is where most owners go wrong, and it is not their fault. The milligrams on the front of the bottle are usually milligrams of fish oil, not milligrams of EPA and DHA. A 1,000 mg capsule commonly contains around 300 mg of combined EPA and DHA. Dosing is done on the EPA and DHA figure, which lives on the back panel.

Amounts scale with body weight and rise for joint support versus general wellness. Rather than invent a number, work from the supplement facts panel and your veterinarian's guidance — VCA Hospitals and the American Kennel Club both publish weight-based references, and Colorado State University's veterinary teaching hospital publishes a canine dosing chart that many vets use.

Start at the low end and build over a week or two. Fat introduced quickly is the most common cause of loose stool, and fish oil is calorie-dense — count it inside the daily food budget rather than on top of it.

How to Give It

Liquid pumped or poured onto food is the easiest format for most dogs and nearly all cats, and it lets you adjust the amount precisely. Softgels work for dogs that take capsules; some owners pierce them and squeeze the oil onto food. Soft chews are convenient but usually deliver less EPA and DHA per serving than the label implies at first glance.

Give it with a meal. Most animals like the taste; a cat that refuses it usually needs a smaller amount mixed more thoroughly rather than a different brand.

Dogs, Cats and Horses Are Not the Same Here

Dogs convert ALA to EPA and DHA poorly, so a marine source does a job a plant oil cannot.

Cats are obligate carnivores with even more limited conversion — a marine source matters more for a cat than for a dog, not less. Cats are also more sensitive to the calorie load, and a fishy oil is often the easiest supplement to get into one. See cat safety for the wider picture.

Horses are traditionally supplemented with flaxseed, which supplies ALA and runs into the same conversion ceiling. Marine-source omega-3 is used in performance and senior horses and is dosed in far larger volumes than for a dog. Talk to your equine vet before adding oil to a feed programme.

Side Effects and Cautions

Fish oil is well tolerated. The usual complaints are loose stool, a fishy smell to the breath and coat, and weight gain if the calories are not accounted for.

At higher amounts there are real cautions worth knowing: omega-3 can affect platelet function, which matters if surgery is scheduled or your animal is on an anticoagulant. Long-term high intake can also increase vitamin E requirements. Discuss sustained high-dose use with your veterinarian rather than deciding it alone.

Human fish oil is chemically the same oil, and the practical problem is different: flavoured liquids and gummy formats may contain xylitol, which is seriously toxic to dogs. Read the full ingredient panel, and dose on EPA and DHA rather than on capsule count.

How to Choose a Fish Oil

Look for the EPA and DHA figures stated per serving on the panel, not just total fish oil. Prefer small, short-lived fish — anchovy, sardine, menhaden — which accumulate less mercury than large predatory species. Look for the National Animal Supplement Council Quality Seal on a pet product, and for a manufacturer that will show you third-party testing; our certificate of analysis entry explains how to read one.

Freshness is the quality marker nobody checks. Fish oil is the most oxidation-prone oil in the cupboard, and a rancid one is worse than useless. It should smell clean and marine, never sharp or paint-like.

Storage: Refrigerate After Opening

Fish oil is highly polyunsaturated, which is exactly what makes it valuable and exactly what makes it oxidise. Refrigerate the bottle once it is opened, keep the cap tight, and keep it out of the light. Buy a size your animal will finish in a couple of months rather than the biggest bottle on the shelf.

The same rule applies to our own seed-oil products — see shelf life and storage for how the different carrier oils compare.

Where Hemp Fits

Fish oil and hemp do different jobs, and we would rather you understood that than bought the wrong one. Fish oil supplies EPA and DHA. Our full-spectrum hemp extract supplies cannabinoids and terpenes in an organic carrier oil, as a daily wellness supplement supporting normal joint function and mobility. Many owners give both; there is no known interaction, and your veterinarian should know about each.

Our own research is a 90-day canine study overseen by veterinary pain-management specialist Dr. Robin Downing with researchers from Colorado State University's College of Veterinary Medicine — read it on our clinical research page, or browse CBD Hemp for Dogs, CBD Hemp for Cats and CBD Hemp for Horses.

Frequently Asked Questions

Is fish oil actually good for dogs? Yes — it is the nutritional supplement with the strongest evidence in the canine joint literature, and it also supports skin, coat, heart and kidney function. It is not a treatment for disease.

How much fish oil can I give my dog a day? Dose on the combined EPA and DHA figure from the back panel, not the total fish oil on the front, scaled to body weight. Start low, build over a week or two, and confirm the amount with your veterinarian.

Can I give a dog human fish oil? Chemically it is the same oil, but check the whole ingredient list first — flavoured and gummy human products may contain xylitol, which is toxic to dogs. Dose on EPA and DHA, not capsule count.

Which fish oil supplement is best for dogs? One that states EPA and DHA per serving, is made from small fish such as anchovy or sardine, carries the NASC Quality Seal, publishes third-party testing, and smells clean rather than sharp.

Is fish oil or hemp seed oil better? They are not competing. Hemp seed oil supplies omega-6 and GLA for skin and coat; fish oil supplies EPA and DHA. If you want long-chain omega-3, only the marine source delivers it directly.

Does fish oil help itchy skin? EPA and DHA support normal skin barrier function, and many owners use it as part of a skin and coat routine. Persistent itching has causes worth diagnosing — see your veterinarian rather than relying on an oil.

Do cats need fish oil more than dogs? Cats convert plant-form omega-3 even less efficiently than dogs, so a marine source is more important for a cat, not less.

Can horses have fish oil? Yes, and it is used in performance and senior horses, though flaxseed is the more traditional equine omega-3. Volumes are far larger than for a dog — ask your equine vet.

Should fish oil be refrigerated? Once opened, yes. It is the most oxidation-prone oil most households own, and a rancid bottle is worse than no supplement at all.

How long before fish oil works? Coat and skin changes show over a full grooming cycle — think six to eight weeks. Joint comfort is judged over weeks of consistent daily use, not days.

Can fish oil be given with hemp? Many owners do and no interaction is known. Tell your veterinarian everything your animal is taking.