The benefits were seen in some breast and bowel cancer patients, with the supplement thought to reduce inflammation.
The research was presented at an American Society of Clinical Oncology meeting in Chicago.
Dr Andrew Epstein of New York’s Memorial Sloan Kettering cancer centre, explained: “It’s adding to the evidence base that nutritionally supplemental therapies – like Omega-3 – may have a place in supporting patients as they go through their cancer care.”
Dr Lisa Wilde, research director of Bowel Cancer UK and Beating Bowel Cancer, said: “This small study explores how fish oil could help improve treatment success and quality of life for some patients with late stage rectal cancer.
“Over half of all bowel cancer patients in the UK are diagnosed at a late stage.
“Furthering our understanding of who might benefit from dietary supplements and at what stage in their treatment could ultimately help improve the lives of those living with and beyond the disease.”
Jane Murphy, clinical nurse specialist at Breast Cancer Care, said: “It’s really interesting to see different avenues are being explored to help women adapt to life after breast cancer, which can be incredibly daunting and difficult.
“Women regularly call our helpline struggling with debilitating side effects from long-term breast cancer treatments, like aromatase inhibitors, and joint pain is a particularly common problem.
“While these drugs are invaluable in helping to stop the cancer returning, they can make moving forward feel like an uphill struggle.
“However, this is a small study so more research is needed before we have clear-cut answers on the benefits of fish oil for easing pain from breast cancer treatment.”