Omega 3 fatty acids are thought to have anti-inflammatory properties, as opposed to omega 6 fatty acids which tend to be pro-inflammatory. The typical American diet for people is high in omega 6 fatty acids and low in omega 3 fatty acids. Because both fatty acids compete to be metabolized in the body, decreasing your intake of omega 6 fatty acids and increasing your intake of omega 3 fatty acids is helpful. However, no one is certain of exactly what the proper ratio of omega 6 to omega 3 fatty acids is. Currently its recommended that it be < 2.5:1.
Other benefits of omega 3 fatty acids include increased blood flow characteristics helping those with heart disease or at risk for stroke. Fish oil may help reduce side effects from chemotherapy. Fish oil may be helpful in inflammatory bowel disease by reducing inflammation. Fish oil may lower triglyceride levels, and "bad" cholesterol. Fish oil may help protect the gastrointestinal tract from damage and side effects of non steroidal anti-inflammatory drugs (those drugs often used for pain control.) Fish oil helps with skin disorders and allergies as well.
Fish oil derived from cold water fish (salmon, mackeral, sardine, anchovy, or herring) is preferred as it results in higher levels of EPA and DHA. Dosing is based on the amounts of EPA and DHA, as well as the pet's condition and risk factors. Plant sources of omega 3 fatty acids have to be metabolized into EPA and DHA and so tend not to work as well as the fish sources. It may take 4-6 months to see improvement with fish oil so I often recommend it be used year-round.
With so meny benefits and very little risk, who can dispute the "wonders" of my fish oil tonic!