Salmon oil is rich in omega-3 polyunsaturated fatty acids, particularly eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which have been investigated for their potential anti-inflammatory effects in rheumatoid arthritis (RA). Multiple randomized controlled trials and meta-analyses have shown that supplementation with fish oil (including salmon oil) can reduce the number of tender joints, duration of morning stiffness, and levels of inflammatory markers in patients with RA. Some studies also report that fish oil supplementation allows for a reduction in the use of nonsteroidal anti-inflammatory drugs (NSAIDs) among RA patients.

The mechanism is believed to involve the modulation of inflammatory pathways: EPA and DHA compete with arachidonic acid, leading to the production of less inflammatory eicosanoids and cytokines. Clinical guidelines, such as those from the American College of Rheumatology, acknowledge that fish oil may provide modest benefit as an adjunct to standard therapy, though it is not considered a replacement for disease-modifying antirheumatic drugs (DMARDs).

Overall, the evidence supports a moderate benefit of salmon oil for symptom management in RA, with its use being scientifically validated but not curative. The effect size is generally modest, and long-term safety appears favorable, though high doses should be used with caution, especially in patients on anticoagulants.