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Just when you thought it was safe to go back to the nutrition store….

Fish oil may reduce the effectiveness of chemotherapy by at least half, a disturbing new study has found.

Led by Dr. Emilie Voest, the medical director of the Netherlands Cancer Institute, the research team found that the fatty acids present in certain fish and fish oil supplements halted the drug's cancer-fighting properties in mice with tumors.

"You lose at least halve the effect of the chemotherapy because of the fish oil," Voest told Time.

Since the research was done on mice, there is little concrete evidence to extrapolate how accurate the theory is for cancer patients of the two-legged variety.

"It's difficult to extrapolate (this research) to humans," says Dr. Lauren Cassell, chief of breast surgery at Lenox Hill Hospital.

Some chemo patients may take fish oil because one study suggested it helps them maintain their weight.

Even if fish and fish oil did have deleterious effects on chemo, patients would only be affected if they were consuming them during chemo treatments, says Dr. David Nanus, chief of hematology and medical oncology at Weill Cornell Medical College and New York Presbyterian Hospital.

"If you want to be safe, don't take the fish oil around the time you're getting chemo," he said. "Don't take it the day of, and the day before and the day after."

Still, he said more research needs to be done before we can say that fatty acids lead to chemo resistance. And Voest said that people who took fish oil while on chemo shouldn't self-diagnose the cause if their chemo treatments weren’t effective.

"Obviously there is a balance between how sensitive a tumor is to chemo versus the resistance-causing effect of fatty acids."

Fish oil isn't regulated by the Food and Drug Administration, so it's impossible to know how much fatty acid is in a supplement.

The study was published in the journal JAMA Oncology.

This article first appeared in the New York Daily News.