Do you wear a fitness tracker to assist you in achieving your health goals? A new warning has been raised by some wearers of Fitbits who have experienced disturbing side effects.

Active dad Chris Nelson enjoyed counting his steps and monitoring his heart rate with his fitness tracker. However, while putting his three-year-old daughter Emily to bed one night, his Fitbit Charge 2 shocked him, causing him to drop the little girl.

“It was clearly an electric shock,” Nelson told CBS2.

Emily was not injured in the incident, but Nelson said that the shock left a painful lump on his wrist and caused his hand to go temporarily numb.

“The best I can figure, I was just shocked by the connector that was on my wrist,” Nelson said.

Lauren Reiss, another fan of fitness trackers, claims that she has sustained shocks from two different Fitbit models.

“It shot up my arm, it was intense,” Reiss said. “As soon as I looked down at my arm I noticed immediately that it hurt.”

According to Reiss, one of the shocks left a lasting scar.

“You can see my burn,” she said. “It matches perfectly.”

Reiss insists that there is a danger associated with Fitbit fitness trackers that must be addressed.

A search of federal complaints reveals additional claims against Fitbit, including one that alleged “it sent a shooting, painful electric shock down to my fingers and up to my elbow.”

Another complainant noted that, after experiencing a shock, the burning sensation lasted a couple of days.

Fitbit claimed that it takes “all reports of potential issues seriously” and its products are “designed to prevent electrical contact with the user.” Fitbit noted that, in the cases the company has investigated, “the devices and batteries were found to be fully intact and functional, with no signs of overheating, voltage irregularities, or malfunctioning of any kind.”

Fitbit suggested that the shocks were likely “caused by static electricity built up on the person.”

Professional engineer Kelly Robinson confirmed that static can build up on a body, but added that it is often cited as a scapegoat when companies cannot discern why their products are malfunctioning.

“I just don’t think it’s static,” Robinson said. “I think there is something else going on.”

Consumer attorney Stuart Talley is concerned about the reports.

“If you had a pacemaker, if you were driving a car, something like that could cause a really serious injury,” he said.

Reiss has a pacemaker, and now chooses to wear her Fitbit farther from her heart, strapping it to her ankle instead of her wrist.

Following the incident in which he was shocked, Fitbit sent Nelson a new, more advanced model. Since then, Nelson has not sustained additional shocks.

If you have experienced a shock from your Fitbit, you are advised to contact the company immediately.