First, talk to your doctor about adding a fish oil supplement. Discuss your dietary habits and ask your doctor to set a daily target of how much omega-3 fatty acids you should be getting through the supplement. Your target also will vary depending on whether you already have coronary heart disease or need to lower triglyceride levels. Mention any other supplements and medications you already are taking, particularly blood thinners. 


Familiarize yourself with what to look for on a fish oil supplement label. Always look for circle on the label that says USP. This indicates that the U.S. Pharmacopeia, and independent lab, has tested and verified the label's claims. You'll also be looking for the acronyms EPA, eicosapentaenoic acid, and DHA, docosahexaenoic acid. These are the key omega-3 fatty acids you'll want from fish oil. The highest benefit can be reaped from fish oil when all eight omega fatty acids are present. The most common include ALA, DHA and EPA. Look for a product that also contains DPA, ETA, HPA, ETA(3) and SDA. To assure all of the oil you are consuming is from omega fatty acids, add up the amount of oil provided by fatty acids and compare that to the total amount of oil in the product. If it comes up short, the other oil could be coming from anywhere. Read the label of fish oil supplements carefully before you choose. If the label says, "fish oil," or "marine lipids," but does not list what fish the oils are derived from, beware. It is best to choose a supplement which is species specific, as anchovies, salmon and sardines are the best sources for omega 3.


Make sure that the manufacturer of the supplement uses fish which have been screened for human consumption, as that is the only way to ensure the oil is free of toxins, especially mercury. Make sure the toxins have been cleaned out of your fish oil supplement by molecular distillation, or that the fish that were the source of the fish oil came from a clean, noncontaminated area of the ocean.If the bottle is not specific about screening, do not hesitate to call the contact number found on the bottle. Choose fish oil supplements that contain both EPA and DHA, as the majority of medical research about the benefits of fish oil are based on both types of fatty acids. If the manufacturer does not specify the types of fatty acids found in the product, choose another product. Be aware that cod liver oil is very high in vitamin A and should only be used daily if you have a vitamin A deficiency. Excess intake of vitamin A can cause dry skin, itchiness, headaches and even liver damage. 


Compare the amount of EPA (Eicosapentaenoic acid) and DHA (Docasahexaenoic acid) in your fish oil supplements. These are the two most important Omega 3 fatty acids. It is important to get the highest possible ratio of EPA to DHA. The standard ratio is 18/12, but there are some supplements that offer a ratio as high as 40/20, which is preferable, as recommended by the Physicians for Optimal Heart and Brain Health, according to Energy First.


Your fish oil supplement should also be fresh. There's an easy test for this: If it makes you burp, it is not fresh, and you should switch to another brand.