Heavy Metals in Fish Oil and Supplements for Cats

The short answer

The research on pet supplements is small, and it points two ways.

A 2026 study of 70 dog and cat supplements sold in the US found lead in 16 and cadmium in 10. Mercury wasn't detected in any of them, and none went over the maximum tolerable levels the authors used. But some products didn't contain what their labels said. In a 2025 study of fish oils for dogs, mercury, lead and cadmium weren't detected in any sample.

Pet supplements are regulated as animal food or animal drugs, not as dietary supplements. Kelp is a separate question, because seaweed can carry a lot of arsenic.

How pet supplements are regulated

In the US, human supplements have their own legal category. A 1994 law, the Dietary Supplement Health and Education Act (DSHEA), created it.

That category doesn't cover pets. According to FDA's page on how it regulates pet food, FDA decided in 1996 that DSHEA doesn't apply to products for animals. Products sold as supplements for pets are regulated as either food for animals or animal drugs, depending on their ingredients and intended use.

So pet supplements aren't unregulated. They just aren't regulated the way human supplements are. Our post on who regulates heavy metals in pet food covers what that means for testing.

What a US study of 70 pet supplements found

The largest study we found was published in 2026 in the Journal of Animal Physiology and Animal Nutrition by researchers at UC Davis. We reviewed its abstract. The team tested 70 supplements for dogs and cats sold in the US for 5 essential minerals and 12 heavy metals.

Here's what the abstract reports:

• Not detected: beryllium, mercury and thallium, in any product.
• Most common heavy metal: nickel, found in 66 of the 70 products.
• Lead: detected in 16 supplements.
• Cadmium: detected in 10 supplements.
• Selenium: an essential nutrient with a narrow margin between enough and too much, detected in 52 products but listed as an ingredient in only 10.
• Label claims: of 18 products that made claims about the minerals tested, 8 missed at least one claim.

None of the 70 products went over the National Research Council's maximum tolerable level for any mineral. The authors said this suggests low risk when owners follow label directions. They also noted that species-specific data are missing for many minerals in dogs and cats, so those limits rely on extrapolation, and that more research is needed on long-term high intake.

Two takeaways stand out. Most products didn't show a heavy metal problem in this study. And a label isn't a test result: several products didn't match their own claims.

Fish oil and mercury

Fish oil worries many owners because fish carry mercury. FDA notes, writing about food for people, that nearly all fish and shellfish contain trace amounts of methylmercury. Our post on fish vs poultry pet food covers mercury in fish-based foods.

Oil isn't the same as the whole fish, though. A 2025 study in Topics in Companion Animal Medicine, by a team that included UC Davis researchers, tested 11 fish oil supplements for dogs, once in 2010 and again in 2021. Molybdenum, cadmium, mercury, lead and PCBs were not detected in any sample. The products did vary in other ways: the omega-3 content varied widely between products and time points, and there were six cases of products not meeting their label claims for the omega-3s EPA, DHA or both.

That study covers dog products only, and 11 products is a small sample. The sources we reviewed did not include a study of heavy metals in fish oil made for cats.

Kelp and seaweed supplements

Kelp is a different story. In a 2012 regulation, EU regulators wrote that seaweed meal and fish ingredients contain a high level of total arsenic, mainly in organic form, which they describe as the less toxic form. Our post on seaweed, kelp and fish meal in pet food explains organic and inorganic arsenic.

The sources we reviewed did not include a study that measured arsenic in kelp supplements made for cats or dogs. So we can't tell you how much a given kelp product carries. Only a test of that product can, and a test that separates the forms of arsenic shows how much is inorganic.

What to ask a supplement maker

The 70-product study shows why it's worth asking. Three questions cover most of it:

1. Do you test the finished product for heavy metals? Ask for lead, cadmium, mercury and arsenic, and for kelp products, arsenic by form.
2. Do you check that the product matches the label? In the US study, 8 of 18 products with mineral claims missed at least one.
3. Which lab ran the tests, and can you share results for this product? Ask for results per product and batch.

Supplements often go into the bowl along with food and treats. Our post on heavy metals in pet treats covers the treat side, and our post on where heavy metals in cat food come from covers the food side.

FAQ

Does fish oil for cats contain mercury? The sources we reviewed did not include a study of fish oil made for cats. In a 2025 study of 11 fish oils for dogs, mercury wasn't detected in any sample, and the US study of 70 dog and cat supplements didn't detect mercury in any product.

Are pet supplements regulated like human supplements? No. FDA says the human supplement law, DSHEA, doesn't apply to animals. Pet supplements are regulated as animal food or animal drugs, depending on their ingredients and intended use.

Do kelp supplements contain arsenic? Seaweed can carry a lot of total arsenic, mostly in organic form, according to EU regulators. The sources we reviewed did not include a study of kelp supplements for pets, so ask the maker for arsenic results by form.

A note on your pet's health

This article is general information, not veterinary advice. Talk with your veterinarian before starting or stopping any supplement.

Supplements can be certified too

Supplement makers can test and certify their products. Heavy Metal Tested is a certification program that helps brands share independent results with owners. Learn how certification works.