An alarming new study has found that smartphones can interfere with pacemakers or other implanted cardiac devices, shocking the user or even causing the person to faint.

Many of us always have our smartphone close at hand, but those of us who have cardiac devices implanted such as pacemakers need to be extra careful, because there’s a small chance it can interfere with the electronics in them, causing all sorts of unpleasant unintended consequences, according to a HealthDay News report.

The study was led by Christof Kolb, who was the former head of the electrophysiology department at the German Heart Centre. Kolb said that although patients who have a cardiac device can use a smartphone, they shouldn’t place it directly over the device — therefore, don’t put it in your pocket right next to where the device operates — and you should hold it to the ear on the opposite side of the device, according to the report.

That’s not to say you should be alarmed and super careful about your smartphone, just be wary of the risks and how to minimize them.

If the smartphone does interview with the pacemaker or other cardiac device, typically it results in a brief pause in function, but even a short pause can cause the patient to faint. In addition, the cardiac device may also see the interference from the smartphone as a life-threatening event that is indicative of an abnormal heart rhythm, and may hit the patient with a painful shock.

The U.S. Food and Drug Administration recommends that cell phones be kept at a distance of at least five to seven inches from a cardiac device that is implanted in the body, but the study is 10 years old and smartphones have come a long way, so it may be advisable to go even a little bit beyond that.

For the study, researchers exposed participants to the electromagnetic field os the Samsung Galaxy 3, Nokia Lumia, and HTC One XL, three popular smartphone brands. They then hooked up the patient and the phone to measure the electrical impulses and see how they interacted.

After 3,400 tests, the study found that only one of hte patients had been affected by interference. This suggests that such interference is quite common, but it can occur, so it’s wise to adhere to current guidelines.