Fish oil supplements are commonly used among chemotherapy patients, but a new study suggests that fish and fish oil supplements could interfere with the effectiveness of chemotherapy.

Researchers looked at the fish oil intake of 118 study participants who were also chemotherapy patients. Of these patients, 35 told the researchers that they use fish oil regularly and 13 used omega-3 fatty acid supplements, according to the study. The team, lead by Dr. Emile Voest from the Netherlands Cancer Institute in Amsterdam, also included 30 healthy volunteers for the fish oil study and 20 health volunteers for the fish study.

The researchers found that all tested fish oils, herring and mackerel contained a chemotherapy-negating fatty acid.

"After ingestion of these fish oils or fishes, 16:4(n-3) was rapidly taken up in the plasma of human volunteers," the researchers wrote. "Until further data become available, fish oil and fish containing high levels of 16:4(n-3) may best be avoided on the days surrounding chemotherapy."

The research was funded by Dutch Cancer Society grant UU-2012-5712. 

"The Dutch Cancer Society had no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication," the study authors wrote. 

The study was published in the April 2 edition of the journal JAMA Oncology.