Many people take vitamin E pills thinking they will help combat lung cancer. A new study suggests that those supplements have exactly the opposite effect in mice, Cleveland Clinic reports.

Researchers in Sweden gave vitamin E supplements in amounts four to 50 times the U.S. recommended daily intake to mice with early lung cancer. The researchers expected the supplements to slow the tumors’ growth, but instead the tumors multiplied and grew more quickly. Results appear in the journal Science Translational Medicine.

Whether those effects would be the same in humans is not proven, says oncologist Nathan Pennell, MD, PhD.

“Although vitamins are essential parts of staying healthy, too much of anything is potentially harmful,” Dr. Pennell says. “Your body is not built to take in incredibly high levels of these vitamins.”

Vitamin E supplements also could interfere with therapies such as radiation and chemotherapy, Dr. Pennell says.