Once used mainly as a vitamin supplement for children, cod liver oil is more likely now to be promoted for older people suffering from severe osteoarthritis. Fish oil, rich in omega-3 fats, helps counter inflammation, so some people with another form of arthritis, rheumatoid arthritis (an inflammatory disease), take fish oil supplements to help reduce symptoms. There is also an inflammatory element in osteoarthritis, it is now known, and thus some people think that fish oil, specifically cod liver oil, might be useful. It's also sometimes claimed that cod liver oil prevents heart attacks, strokes, Alzheimer's disease and depression, and fights cancer.
No solid research has ever shown that cod liver oil prevents cancer, Alzheimer’s or depression, but there is some evidence that fish oil, including cod liver oil, may ease the pain of arthritis and improve joint stiffness. Like all fish oil supplements, the oil may help reduce the risk of heart disease and stroke, especially in people who have already had a heart attack or are at high cardiovascular risk.
Potential cod liver oil problems
There have been concerns that cod liver oil, bottled or in capsule form, can weaken bones and cause birth defects because of its very high levels of vitamin A. For example, a Norwegian study of middle-aged women, published in the American Journal of Epidemiology, found that those who took cod liver oil as children are twice as likely to have low bone mass as women who did not.
Although research has been inconsistent, some studies have found that as little as 6,000 IU of vitamin A daily can interfere with bone growth and promote fractures. And women of childbearing age should not exceed 10,000 IU of vitamin A because of the risk of birth defects. Just one teaspoon of cod liver oil may have 4,500 IU of vitamin A, and the standard dose is one to three teaspoons a day. Capsules of cod liver oil contain less oil, but the vitamin can still add up, especially on top of the amount you get from foods and from a multivitamin, if you take one.
Another potential problem: Since the oil is made from livers, which filter out toxins, there is greater concern about contaminants (such as PCBs), even though the oil is supposed to be purified.
Bottom line: Don't take cod liver oil, unless the label says that the vitamin A content has been reduced. Fish oil supplements, and fish itself, do not have much vitamin A, which is concentrated in the liver (in beef and chicken liver as well).
Originally published October 2007. Updated November 2012.