Can fish oil supplement hinder cancer treatment?

Fish oil is among the top-selling nutritional supplements in the U.S. and has long been under the microscope regarding its effectiveness. Now a new study is raising questions about the oil's safety.

Researchers at the Netherlands Cancer Institute in Amsterdam found consuming particular fatty acids in fish could significantly impede the effects of chemotherapy, said CBS News medical contributor Dr. David Agus.

"So when there's never been shown a benefit to patients with cancer, of fish oil capsules, and there's a potential negative, patients right now should avoid it," Agus said Friday on "CBS This Morning."

Data from 118 cancer patients undergoing chemotherapy and 50 healthy volunteers were compared to results collected from an experiment performed on mice. Thirty-five cancer patients reported regularly taking fish oil supplements, and 13 used supplements containing omega-3 fatty acids.

Agus said the data revealed human patients could get the same fatty acid levels in their blood as the mice in the experiment. He said it would be unethical to repeat a similar experiment on human patients undergoing treatment for cancer, citing the potential to inhibit chemotherapy.

Fish oil supplements may also be dangerous for healthy patients. According to the National Institutes of Health, possible side effects when taking high doses can include, nausea, lowered immune response and, according to Agus, there has been a study that showed a potential increase in prostate cancer.

Representatives of the supplement industry insist fish oil capsules can be safe.

"Rigorous safety evaluations by government authorities have shown that 3-5 grams of omega-3 fatty acids daily is safe for adults," said Duffy MacKay, senior vice president of scientific and regulatory affairs for the Council for Responsible Nutrition.

Nevertheless, Agus still advises his patients against it.

"I want you to eat real food," he said. "I want you not to take fish oil capsules and avoid mackerel and herring, which have the highest amounts of this particular fatty acid."