Polyunsaturated fatty acids (PUFAs) are classified as omega-6 or omega-3, usually expressed at the dietary ratio of n6:n3, although the total n6:n3 ratio should be used cautiously as it does not represent the total amount or type of omega-3 fatty acid present. Supplementation with eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), the omega-3 fatty acids found in fish oil, is often recommended for management of diseases (eg, neoplasia; hyperlipidemia; dermatologic, cardiovascular, renal, GI, orthopedic diseases). Target doses typically range from 50–220 mg/kg with higher dosages often used to lower serum triglyceride concentrations and lower dosages used for inflammatory conditions, renal disease, and cardiac disease. Potential adverse effects of supplementation include lipid peroxidation, nutrient excess and toxin exposure, nutrient–drug interactions, weight gain, altered platelet and immune function, detriments to wound healing, adverse GI effects, adverse effects on glycemic control, and insulin sensitivity. Adverse effects are likely dose dependent. The current safe upper limit for combined amounts of EPA and DHA is 2,800 mg/1,000 kcal diet or 370 mg/kg for dogs. Limited published data prevent a safe upper limit recommendation for cats. Clinicians should understand the adverse effects that may occur with omega-3 fatty acid supplementation; risks should be balanced with potential benefits.

Commentary
The authors reviewed nearly 100 publications in order to summarize risks of fish oil supplementation. Clinicians tend to focus only on benefits of EPA and DHA, but supplementation may lead to unexpected effects. Numerous nutraceuticals and commercial pet foods contain high levels of fish oil, underscoring the need for awareness of EPA and DHA doses and possible adverse effects.—Craig Datz, DVM, MS, DABVP (Canine & Feline), DACVN

Source
Potential adverse effects of omega-3 fatty acids in dogs and cats. Lenox CE, Bauer JE. JVIM 27:217-226, 2013.