The best type of cod liver oil will depend on why you're interested in taking this supplement, as each type has its benefits and disadvantages. Cod liver oil provides omega-3 fats as well as vitamins A and D. Different supplements provide different amounts of these nutrients, and different formulations can help with other potential issues, such as side effects and palatability. Speak with your doctor before taking cod liver oil to make sure it would be safe for you.
Check for High Omega-3 Content
A tablespoon of cod liver oil provides about 2,894 milligrams of total omega-3 fats, including 2,502 milligrams of combined eicosapentaenoic acid and docosahexaenoic acid, the two omega-3 fats most recommended for improving health. Cod liver oil typically contains less EPA and DHA than other fish oil supplements, which have sometimes been purified to maximize their EPA and DHA content.
Look for a supplement containing 450 milligrams of EPA plus DHA per day, recommends Dr. Carrie Ruxton of the Health Supplements Information Service in a Daily Mail article published in July 2012. This is not the same as the amount of total fish oil contained in the supplement -- a supplement might have 1,000 milligrams of total fish oil but only 450 milligrams of DHA and EPA -- so check the label to determine the EPA and DHA content.
Look for Lower Vitamin A Content
Cod liver oil can be very high in vitamin A, which is stored in the body when consumed. The high amounts of vitamin A in cod liver oil make it easy to overdose on vitamin A. For example, a teaspoon of cod liver oil has 5,000 international units of vitamin A, or 100 percent of the daily value. Adults shouldn't consume more than 10,000 international units per day, which is the tolerable upper intake level, so just 2 teaspoons provide the maximum safe amount vitamin A per day -- not to mention the other sources of vitamin A in your diet or other supplements. Due to these concerns, some cod liver oils contain reduced amounts of vitamin A, making vitamin A toxicity less likely.
Avoid Oils With PCBs
Although you don't need to choose the most expensive type of cod liver oil, you should choose a reputable brand to limit the potential risk of contaminants, such as mercury or PCBs. Mercury, PCBs and other contaminants can build up in your body and increase your risk for certain health problems, including cancer and having a child with birth defects. Those with the United States Pharmacopeia symbol tend to be good choices, as they have been tested for contaminants and to see if they contain the ingredients stated on the label.
Some people find it difficult to swallow liquid cod liver oil, in which case capsules or flavored varieties of cod liver oil may be preferable. Enteric-coated capsules can help limit the risk for gastrointestinal side effects that sometimes occur with fish oil supplementation. Keep in mind that capsules tend to be more expensive than liquid cod liver oil and that you may need to take multiple pills to get the recommended amount of omega-3 fats. Colorado State University Extension recommends choosing fish oil capsules that contain at least 50 percent omega-3s -- DHA and EPA -- in each capsule.
Fish oil, including cod liver oil, can cause side effects including bad breath, headaches, upset stomach, a metallic taste and dry skin. Switching to a different brand, spreading your fish oil intake throughout the day instead of taking a large dose at one time and taking your supplement with food may help improve absorption and limit side effects.
People who are allergic to fish and those with certain health conditions, including kidney or liver disease, stomach problems, high blood pressure, diabetes and high cholesterol, shouldn't take fish oil supplements. Pregnant women should also avoid cod liver oil due to its high vitamin A content.
Cod liver oil may interact with some medications, including blood thinners, vitamins A and D, medicines for kidney disease, medicines for bone health, orlistat, colestipol and cholestyramine.