A patch that slowly delivers nicotine, pain medication or other drug through the skin can cause a skin burn resembling a bad sunburn when worn during an MRI scan, U.S. regulators said on Thursday.

The Food and Drug Administration advised people not to wear drug patches, about a quarter of which have small amounts of aluminum or other metal that may overheat and burn the skin, during a magnetic resonance imaging scan.

Dr. Sandra Kweder of the FDA's Center for Drug Evaluation and Research said there have been "fewer than half a dozen reports" of people experiencing "a severe sunburn-like reaction" while wearing a metal-containing patch in an MRI.

They had been wearing nicotine patches, which are used to help people quit smoking, Kweder said.

There are about 60 individual patch products, about three quarters of which have adequate warnings, she added.

"The problem is the patient can't necessarily tell while they're wearing the patch if the patch has metal in it or not," Kweder told reporters during a conference call.

The metal in the patches may not be visible and the product labeling may not disclose its presence, the FDA said. Kweder said there is no standard warning on patch products stating whether they are safe in an MRI.

The FDA's Eric Duffy said the agency "simply neglected to ensure that that label was there" on all applicable products.

The FDA said it was alerted to the missing MRI warning on generic drug maker Teva Pharmaceutical Industries Ltd's fentanyl painkiller patch in January, and then found that warnings also were missing on a variety of patches.

About three quarters of drug patches, some available by prescription and others over the counter, contain adequate warnings, and the FDA is working to make sure all of them do, officials said.

MRI imaging uses a magnetic field and radio waves to get detailed images of organs and tissues looking for things like tumors, injuries and various abnormalities.

The metal contained in the patches, while not attracted to the magnetic field of the MRI, can conduct electricity, generating heat that can cause burns, the FDA said.