Cyberonics Inc.‘s request to market an implantable nerve stimulator for treating chronic depression should be rejected by U.S. authorities, consumer group Public Citizen said on Wednesday.

In a letter to the Food and Drug Administration, the group said Cyberonics had not proved the device was effective or safe for depressed patients.

The company’s studies provided little evidence that the device helped relieve chronic depression, Public Citizen said. Plus, concerns about worsening depression, suicide attempts and sudden deaths in the trials have not been fully investigated, the group said.

“With so many uncertainties and red flags, it is a serious mistake for the FDA to be prepared to approve this device for use in millions more people for whom it has not been proved to work,” the letter said.

“Do not let justified empathy for this patient population lead to the unjustified approval of a device that does not come close to meeting FDA’s approval standards, and may well do more harm than good,” the group also said.

The Cyberonics product, known as the VNS Therapy System, is an implantable stopwatch-size device that sends electrical pulses to the vagus nerve in the neck. The product is approved for helping to control epileptic seizures, and Cyberonics is seeking permission to market it for cases of depression that persist despite therapy with antidepressant drugs.

The only study that compared the VNS system to a “sham” treatment - a device that provided no stimulation - failed to show a benefit for the therapy, Public Citizen said.

Another analysis did show depression improved, but because there was no comparison group it is impossible to tell if the device was responsible, Public Citizen said, citing an analysis by an FDA statistician that called Cyberonics’ conclusion “highly questionable.”

In August 2004, the FDA turned down Cyberonics’ application for use of the device for depression. The agency had concerns about both the safety and effectiveness of the device, Cyberonics said at the time.

The FDA reversed the decision in February and told Cyberonics it would approve the treatment if certain conditions were met. Cyberonics has said it expects full approval this month.

The VNS device is the only product Cyberonics sells. Annual sales are about $110 million for epilepsy treatment, but analysts estimate the market for depression is much larger.