Fish oil side effects in dogs

By the PetFishOilGuide editorial team · Updated October 13, 2026 · How we compare

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Reported fish oil side effects in dogs include loose stool, vomiting, weight gain, and possible effects on blood clotting and wound healing. Veterinary reviewers describe them as likely dose-dependent. Stop and call your vet if your dog has vomiting, diarrhea, or unusual bleeding or bruising, and tell your vet about fish oil before surgery.

Fish oil is common, but common is not the same as risk-free. This page summarizes what veterinary sources say can go wrong, so you know what to watch for. It is a source summary, not veterinary advice, and has not been checked by a veterinarian. For amounts, see the dosage calculator, which shows only the suggested supplemental range for adult dogs and the NRC upper limit.

What the veterinary literature lists

The main source is a 2013 review of omega-3 adverse effects by two veterinarians, Lenox and Bauer, in the Journal of Veterinary Internal Medicine, summarized by dvm360. A 2016 article by Lenox in Today's Veterinary Practice and a 2020 article by Raditic and Gaylord in the same journal list similar concerns: drug-supplement interactions, altered platelet function, gastrointestinal disturbances, effects on wound healing, weight gain, altered immune function, and effects on insulin sensitivity. The 2013 review describes adverse effects as likely dose-dependent. The sections below take the ones owners ask about most.

Stomach and bowel upset

Diarrhea and vomiting are the effects Colorado State University's veterinary hospital names in its dosing chart. The dvm360 summary says diarrhea may occur from poor fatty acid digestion, and that slow dose transitions can help. CSU's advice for vet-supervised dosing is to start low and increase slowly while watching for side effects. For an owner, the practical points are:

• Use the lower end of the published range first, not the higher end. We explain the range on the dosage page.
• Do not start a new fish oil on the same day as other new foods or treats, so you can tell the cause.
• If stool stays loose, or your dog vomits more than once, stop the supplement and call your vet.

Some owners notice fishy breath. We could not find a veterinary source that measures it, so we make no claim about how often it happens. CSU does recommend choosing products tested for purity and freshness.

Weight gain

Oil has calories. The dvm360 summary points out that supplement calories count toward your dog's daily intake, which matters for overweight dogs and breeds prone to weight gain. If your dog is already on a weight plan, ask your vet whether to cut food to balance the added oil.

Blood clotting, drug interactions and surgery

EPA and DHA can affect how platelets work. The dvm360 summary says that studies in the veterinary literature so far have shown little if any clinical impact, but it also says supplements may worsen the adverse effects of drugs that affect platelets, such as aspirin, NSAIDs or clopidogrel. In an unrelated study cited by Lenox, omega-3 and glucosamine supplements were suggested as a possible contributor to chronic blood loss in dogs with a certain blood condition. For that reason, the practical rules are:

• Tell your vet about fish oil whenever your dog is prescribed a new medication, especially a pain medicine or blood thinner.
• Do not give fish oil on your own to a dog with a bleeding or clotting problem.
• Before surgery or a dental procedure, tell the clinic. The review states that caution should be exercised with these supplements in the perioperative period, because anti-inflammatory effects could delay wound healing, although research has not shown this so far.

Pancreatitis, diabetes and high blood fats

The 2013 review reports no known cases of fish oil causing pancreatitis in dogs, cats or humans. That does not mean the oil is a good idea for every dog with pancreatitis or high blood fats, because it adds fat. Ask your vet. The same review advises caution with these products in dogs with diabetes mellitus, since insulin sensitivity may be affected.

Too much, and how the upper limit works

The National Research Council set a safe upper limit of 2,800 mg of combined EPA+DHA per 1,000 kcal of diet, which veterinary authors convert to about 370 mg x (kg of body weight)^0.75 for an adult dog. It includes the EPA+DHA already in the food. Higher doses used in studies are chosen and monitored by veterinarians. Our calculator only shows the suggested supplemental range and never an amount above it. The arthritis page lists higher study and chart values for reference only; those are for veterinary supervision, not for home use. The limit is background information: it includes the EPA+DHA already in the food, so count the food too.

The review also mentions oxidation of the oil, which can use up vitamin E, and a small possibility of heavy-metal exposure from fish oils, which it describes as low risk. Fresh, tested products and sealed, cool storage address both. See how we compare products for what we record and what we cannot see.

When to call your vet

• Vomiting more than once, or diarrhea lasting more than a day.
• Black or bloody stool, nosebleeds, or unusual bruising.
• Loss of appetite, tiredness, or belly pain.
• Any reaction when your dog is on prescription medication.
• Scheduled surgery, dental work, or pregnancy.

If your dog is in distress, call your vet or an emergency clinic now rather than waiting. More background is on omega-3 for dogs and human fish oil for dogs.