Fish oil supplements might make cancer chemotherapy less effective — but many people with cancer were taking those supplements in a recent survey, reports Reuters.

All six of the fish oil supplements the researchers tested contained a specific fatty acid that's been found to reduce the effectiveness of chemotherapy in mice, the researchers report in JAMA Oncology.

People receiving chemotherapy should refrain from taking fish oil supplements and discuss any supplement with their doctors, said Dr. Emile Voest, the study's lead author from the Netherlands Cancer Institute in Amsterdam.

People should be cautious about going overboard with avoiding all omega-3 fatty acids, because they are an essential part of the diet that the body cannot make on its own, said Stacy Kennedy, an expert on oncology nutrition at the Dana-Farber Cancer Institute in Boston.