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For dogs and cats · May 5, 2021

Fish oil for pets is overdue a serious rethink

Your dog or cat genuinely needs EPA and DHA, but the fish oil most people reach for comes with a contamination problem the industry would rather you didn’t notice, and there’s already a cleaner way to get those same fatty acids in.

Golden retriever splashing through a sunlit pond — fish oil EPA and DHA for dog coat and joint health

By Beryl Shuttleworth, BSc Hons Biochem

Last reviewed: 2026-06-12

Fish oil is well-proven for dogs and cats, good evidence backs it for skin, joints, heart, and brain health. The catch is contamination: fish oil regularly contains PCBs, pesticide residues, and heavy metals, and how much depends heavily on where the oil comes from and how well it’s been purified. Algal oil gives you the same EPA and DHA, but because algae sit right at the bottom of the food chain, all that toxic baggage hasn’t had a chance to build up. It’s also been confirmed safe across multiple life stages in both dogs and cats.

Fish oil is always the best source of EPA and DHA for dogs and cats

I get why this one sticks. Fish oil has decades of vet research behind it, it’s easy to find, and the EPA and DHA in it genuinely do work in dogs and cats. That’s not up for debate.

But “fish oil works” and “fish oil is the best source” are two very different statements, and the second one is worth a harder look.

Here’s the thing most people don’t know: fish didn’t make those omega-3s. They got them by eating phytoplankton and tiny single-celled algae, because fish simply don’t have the biological kit to make EPA and DHA themselves. The marine food chain is just the go-between, ferrying those fats from the original algal source all the way to the bottle on your shelf [5]. And every step up that food chain? Another opportunity for contaminants to build up.

Fish oil products have been found to contain persistent organic pollutants, things like PCBs, dioxins, and organochlorine pesticides, along with heavy metals such as mercury, lead, and cadmium [6][7][8]. Microplastics make things worse: they soak up PCBs and similar pollutants at concentrations far higher than the surrounding seawater, and those nasties travel right up through the food chain [4]. Of the fish oil supplements that have been tested, the unrefined, unpurified ones carried notably higher PCB and organochlorine levels than oils that had been molecularly distilled from smaller fish like anchovies and sardines [3].

The good news is that proper processing does make a real difference. Molecular distillation and activated carbon filtration can strip out 82-95% of dioxins from fish oil [6], so a well-processed oil is genuinely cleaner than a crude one. But it matters which product you pick, and that’s exactly my point.

How EPA and DHA work, and where they actually come from

EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid) are the two omega-3 fatty acids that actually do the work, helping to manage itchy skin, stiff joints, heart health, and brain function in dogs and cats. You may also see ALA (alpha-linolenic acid) listed on supplement labels, but ALA is just a starting point; it’s not the active form your pet’s body uses.

Here’s something the fish oil industry rarely mentions upfront: fish don’t actually make EPA and DHA themselves. They can’t, they simply don’t have the right biological kit for it. The real source is phytoplankton and single-cell algae at the bottom of the ocean food chain. Fish just accumulate these fatty acids by eating their way up from that algal base.

Why does that matter? Because pollution builds up at every step of that food chain. Fish oil products carry measurable amounts of persistent organic pollutants, things like PCBs, dioxins, and organochlorine pesticides, plus heavy metals such as mercury, cadmium, and lead. Microplastics make things worse: in the ocean, they soak up PCBs and other nasties at concentrations far higher than the surrounding water, then pass those contaminants along as they move up the food chain. If you are going to use fish oil, molecularly distilled oils made from anchovies and sardines tend to have the lowest contamination levels; non-purified oils carry substantially more.

Phytoplankton like Nannochloropsis sidestep this problem altogether, because it comes straight from the bottom of the food chain rather than the top. The contaminants simply haven’t had the chance to build up. Also, the phytoplankton are grown in clean filtered seawater with zero contamininants.

How to use EPA/DHA supplementation in dogs and cats: what the evidence actually supports

Minimum requirements, growing and reproductive dogs

AAFCO has set a dietary minimum of 0.05% EPA + DHA (dry matter basis) for growing and reproductive dogs, this is the level the science says you need to support nervous system development and healthy vision in puppies and newborns. The safe upper limit sits at 11 g/kg of diet (1.1% by weight). Stay within that range and you’re on solid ground.

Cats

The research confirms EPA/DHA benefits for cats too, feline osteoarthritis in particular, and AAFCO minimum and maximum figures exist for cats during gestation, lactation, and growth. I don’t have cat-specific therapeutic dosage ranges to share here, so I won’t make them up.

Timing and duration

The clinical trials behind the systematic review showed real benefits across skin, joint, and heart conditions, but the evidence doesn’t tell us the ideal time of day to give these supplements, or exactly how long before you’d expect to see results. Rather than guess, I’ll leave that question open.

The form you give matters

Not all omega-3 sources are built the same. Phospholipid-conjugated EPA and DHA, the kind found in krill oil and certain microalgal sources including *Nannochloropsis oculata*, are better absorbed than the triglyceride form used in most standard fish oils. In plain terms: a smaller dose from a phospholipid-rich source can put more EPA into your pet’s bloodstream than the same number of milligrams from a bog-standard fish oil capsule. We don’t yet have the full picture of how much this matters clinically in dogs and cats specifically, but it’s a real difference worth knowing when you’re comparing products.

When to be cautious, what the evidence actually flags

This is the section most supplement articles quietly skip. I won’t.

The veterinary literature documents the following adverse effects with omega-3 supplementation in dogs and cats:

– *Altered platelet function*, worth knowing before any surgery, or if your animal has a clotting problem
– *Digestive upset*, it’s reported, though we don’t have a clear dose threshold
– *Slower wound healing*
– *Lipid peroxidation*, a particular concern with poor-quality, unprotected oils
– *Interactions with medications*, the research flags this category; if your pet is on any drugs, talk to your vet
– *Effects on blood sugar and insulin sensitivity*, relevant if your animal is diabetic
– *Weight gain*, fish oil is calorie-dense, and that’s not a small thing if your dog is already carrying extra weight
– *Changes to immune function*
– *Risk of nutrient excess and toxin exposure* at high doses, or with contaminated sources

Source quality is not a nice-to-have

Molecularly distilled fish oils made from anchovies and sardines consistently show the lowest levels of PCBs and organochlorine pollutants among tested products. Non-purified fish oils carry substantially higher contamination. Activated carbon processing can strip out 82-95% of dioxin-type compounds, but only 26-45% of dioxin-like PCBs. Processing helps; it doesn’t solve the problem entirely.

There’s another layer here too. Fish-based diets already carry meaningfully higher arsenic, cadmium, and mercury loads than poultry- or red meat-based diets. If your pet is eating a fish-based food and you add a fish oil supplement on top, you’re stacking that exposure. A microalgal source like *Nannochloropsis*, grown in clean, controlled conditions, sits right at the bottom of the food chain and doesn’t carry the same toxic baggage, though its EPA content per gram is lower than you’d get from a concentrated fish oil.

What we’d recommend


Herbal Pet Omega 3 Formula

Clean plant-sourced omega-3 EPA for your pet, none of the ocean pollutants fish oil brings along.

See the product

Frequently asked questions

What are EPA and DHA actually used for in dogs and cats?

EPA and DHA are the two omega-3 fatty acids that have real, evidence-backed uses in dogs and cats, things like itchy or inflamed skin, stiff joints, heart health, brain function, and behavioural issues. One thing worth knowing: how much you need depends entirely on what you’re trying to help. The amount needed for joint support, for example, is quite a bit higher than what you’d use for skin conditions, so it really does matter what you’re targeting.

Is fish oil contaminated, and how serious is the risk?

Honestly, yes, contamination in fish oil is well documented. We’re talking about measurable levels of PCBs, dioxins, pesticide residues, and heavy metals like mercury, cadmium, and lead. Mercury levels in fish alone can range from 0.07 to 1.56 µg/g depending on the species. The good news is that oils made from small fish like anchovies and sardines, and processed using molecular distillation, tend to come out much cleaner. Activated carbon filtration can strip out 82-95% of dioxins, but not every product tells you how it’s been processed, and that matters.

Why do microplastics in the ocean make fish oil contamination worse?

Microplastics work like tiny sponges for pollutants. Floating in seawater, they soak up PCBs, PAHs, and pesticide residues at concentrations far higher than in the water around them, in fact, studies show they can absorb 20-60% of PCBs in just three weeks. When sea creatures eat those plastic particles, everything hitching a ride on them gets passed along. And the further up the food chain you go, the more concentrated that load becomes, which is exactly why oil from larger, longer-lived fish tends to carry the heaviest burden.

Do fish actually make EPA and DHA themselves?

Not at all, fish can’t actually make EPA and DHA. They don’t have the enzymes for it. Instead, they get these fatty acids by eating phytoplankton and single-cell algae, which are the true original source. Fish are really just a middleman. That’s why algal oil, which cuts out that middleman entirely and goes straight to the source, isn’t some nutritional workaround. It’s simply a more direct route to the same fatty acids.

Is algal oil safe for puppies, kittens, and pregnant animals?

The safety studies are reassuring. Research covering dogs and cats through gestation, lactation, and growth up to 26-32 weeks found no concerning changes in blood chemistry, organ function, or clotting compared to control groups. For dogs, AAFCO sets the safe upper limit for combined EPA and DHA at 11 g per kg of diet, a sensible ceiling to be aware of, but one that well-formulated algal oil products sit comfortably within.

Sources

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