Fish oil contains EPA (eicosapentaenoic acid) and DHA docosahexaenoic acid), both omega-3 fatty acids

Omega-3 fatty acids, together with the omega-6 fatty acids, are important components of a healthful diet. The body cannot manufacture them and must obtain them from grains, fruits, vegetable oils, and other foods. In addition, people should consume a balanced ratio of omega-6 and omega-3 fatty acids. Some researchers believe that these two types of fatty acids should be consumed in a 1:1 ratio, while others maintain that people should obtain several times more omega-3 than omega-6 fatty acids from their diet. In either case, the fact that fish oils are high in omega-3 fatty acids may help people to maintain a good balance between the two types of fatty acids.

EPA and DHA in turn are important components of cell membranes and as such play a vital role in overall health.  Studies have shown that Greenland Eskimos, who have a very high intake of fish oils, have a very low incidence of heart attacks, asthma, diabetes, psoriasis, and allergies.

Fish oils have demonstrable benefits in the prevention and treatment of cardiovascular disease; they reduce blood pressure and triglycerides, modify platelet aggregation, and minimize inflammatory responses.

Fish oils are found in high concentrations in herring, sardines and anchovies, in medium concentrations in salmon, and in somewhat lower concentrations in sole, halibut, cod and shellfish.

Health Concerns
High Triglycerides, Heart diseases, Hypertension, Rheumatoid arthritis

Warnings
Fish oil generally appears to be safe when taken as a dietary supplement. The most common side effects are mild indigestion or a fishy taste in the mouth.

Fish oil supplements may interact with aspirin, nonsteroidal anti-inflammatory drugs (NSAIDS), warfarin, or other anti-clotting medications to cause excessive bleeding.