A research paper, released Wednesday, says pacemakers from three major companies—Medtronic, Abbott, and Boston Scientific—were all susceptible to magnetic interference, which could be deadly, as it may cause the devices to work incorrectly.

“Apple’s iPhone 12 Pro Max MagSafe technology can cause magnet interference on CIEDs [more commonly known as pacemakers] and has the potential to inhibit lifesaving therapy,” the AHA wrote.

MagSafe is Apple’s wireless charging technology that uses magnets to quickly charge devices. The number of magnets in the iPhone 12 is greater than in earlier versions of the phone.

Researchers placed iPhones very close to 11 different pacemakers and defibrillators, some implanted in patients and others not. Three of the internal devices registered magnetic interference. Eight of the external ones did.

That’s notable since some users carry their iPhone in their breast pocket. And if someone uses a pacemaker that is impacted by the magnetic interference, it could “lead to asynchronous pacing or disabling of anti‐tachycardic therapies,” the AHA report said.

The AHA noted that the sample size tested was small and recommended that a larger-scale study be performed to confirm the findings. Apple, in a statement made two months ago, suggested people who have a pacemaker keep their iPhone and MagSafe accessories more than six inches from their device.

Heart experts agreed and said it may be wise to consult a doctor if patients have one of these phones.

“Although the Food and Drug Administration website states that cell phones do not pose a significant health risk for patients with these devices, they do acknowledge that certain precautions may be advisable,” the AHA study said. “Based on the variability of interactions with respect to different smartphone models, patients are advised to consult with a heart rhythm specialist regarding recommendations specific to their smartphone and CIED.”