Fitbit has announced a voluntary recall of the Fitbit Force after a number of users complained of somewhat mysterious bouts of skin irritation, usually occurring a number of weeks after they began wearing the tracker.

"Recently, some Force users have reported skin irritation," Fitbit CEO James Park said in an open letter to customers. "While only 1.7 percent of Force users have reported any type of skin irritation, we care about every one of our customers. On behalf of the entire Fitbit team, I want to apologize to anyone affected."

Fitbit does not appear to have isolated the cause of the reaction, which was initially thought to be an allergic reaction to nickel in the bracelet's fastener. A recent report in Consumer Affairs points out a number of inconsistencies between peoples' reaction to the Force and the typical nickel allergy, however, including the time frame and location of the rashes, and the fact that one person reported reacting despite wrapping the device in electrical tape.

"All Force materials are commonly used in consumer products," Park wrote. "However, some users may be reacting to the nickel present in the surgical grade stainless steel used in the device. Other users are likely experiencing an allergic reaction to the materials used in the strap or the adhesives used to assemble the product."

Perhaps because the cause is still unclear, Fitbit is not offering to replace users' Force wristbands with a new device. Instead, Force customers who send in their device can receive a full refund on the device. Park said in the same letter that Fitbit's next generation device is coming soon, implying that the company will simply be moving on with its product pipeline, rather than trying to fix the Force. Additional Fitbit Forces will not be sold by the company.

Fitbit is not the first activity tracker company to issue a voluntary recall and discontinue a product. In 2011, Jawbone experienced an early stumble in the space, when it voluntarily recalled the first generation of its UP bracelet in 2011 following reports of numerous electronic problems. That recall set the company back a full year, almost sinking Jawbone's health tracker business before it began.

The news comes at a time when Fitbit's profile is quite high: Last month, the NPD group released data indicating that Fitbit is leading its competitors by a significant margin in retail sales, making up 67 percent of the full-body activity trackers sold in 2013.