FitBit Inc.'s heart monitoring devices are called "wildly inaccurate" in a lawsuit accusing the company of false advertising.

The claimants are also seeking monetary damages, because they believe they were coaxed into purchasing these malfunctioning fitness trackers, and therefore the money they lost should be promptly returned.

The lawsuit alleges that trackers such as the Fitbit Charge HR: "do not and can not consistently and accurately record wearers' heart rates during the intense physical activity for which Fitbit expressly markets them".

After discovering that her Charge HR did not work as promised, one plaintiff said she complained to Fitbit and sought a refund but received nothing. But investors were clearly not impressed with the new product and shares for the company fell by a dramatic 18%.

New smartwatch introduced by Fitbit created to rival Apple was introduced on Tuesday. Algorithms then determine heart rate, which, according to the plaintiffs, tends to be considerably off the mark. The actual heart rate was recorded, subsequent to performing intense exercises, by her personal trainer. The other plaintiffs make similar claims. According to the lawsuit, this was "approaching the maximum recommended heart rate for her age, and if she had continued to rely on her inaccurate PurePulse Tracker, she may well have exceeded it, thereby jeopardizing her health and safety". However, the company claims its product is an exclusive fitness device and not a smartwatch, as it offers no support for third-party apps, unlike Apple's product, and can pair with multiple operating systems, unlike the Apple Watch.

"Fitbit stands behind our heart rate technology and strongly disagrees with the statements made in the complaint and plans to vigorously defend the lawsuit", the statement reads. "Our team has performed and continues to perform internal studies to validate our products' performance", the statement said. In a counterstatement, the gadget-maker said the case holds no merit and its heart-rate technology functions properly.

"PurePulse continuous heart rate shows you how hard you're working", a Fitbit ad announced.

A company spokesperson told the outlet: "It's... important to note that Fitbit trackers are created to provide meaningful data to our users to help them reach their health and fitness goals, and are not meant to be scientific or medical devices".