Effect of cod liver oil on symptoms of rheumatoid arthritis
Adv Ther. 2002 Mar-Apr;19(2):101-7.
Gruenwald J, Graubaum HJ, Harde A.
PhytoPharm Consulting, Institute for Phytopharmaceuticals, Berlin, Germany.

Abstract
In this pilot study, 43 patients with rheumatoid arthritis ingested 1 g of cod liver oil (one capsule) daily for 3 months. Decreases occurred in morning stiffness (52.4%; P<10(-3)), painful (42.7%) and swollen (40%) joints (P= 10(-3) each), and pain intensity (67.5%). Ratings of “good” or “very good” were awarded by 68% of the patients for efficacy and by 95% for tolerability. Eleven patients reported nonsevere adverse effects that in 3 cases may have been related to the study preparation. Cod liver oil can be recommended for the treatment of rheumatoid arthritis.

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Source: Wikipedia & WebMD

Cod liver oil is a nutritional supplement derived from liver of cod fish. As with most fish oils, cod liver oil has high levels of the omega-3 fatty acids, EPA and DHA. Cod liver oil also contains vitamin A and vitamin D. It has historically been taken because of its vitamin A and vitamin D content. It was once commonly given to children, because vitamin D has been shown to prevent rickets and other symptoms of vitamin D deficiency.