Smartwatches and smart rings that claim they can measure blood sugar on their own, without piercing the skin, have not been approved by the U.S. Food and Drug Administration. And the regulatory agency is warning consumers not to trust them to do that.

In an announcement Wednesday, the FDA said that it “has not authorized, cleared or approved” such a device to measure or estimate blood glucose values on its own.

The warning does not apply to smartwatch applications that connect to and display data from devices that pierce the skin to measure blood sugar and which have been approved by the agency, the notice said.

The FDA is warning that the devices are made by dozens of companies and sold under many brand names. “This safety communication applies to any smartwatch or smart ring that claims to measure blood glucose without piercing the skin, regardless of manufacturer or brand.”

Measuring blood glucose is key to managing diabetes but relying on devices not proven to be accurate can cause errors that could be dangerous or even deadly, the agency said,

About diabetes

Diabetes is a chronic disease where the body doesn’t turn food into energy properly. The Centers for Disease Control and Prevention says 37.3 million Americans have diabetes while 96 million have prediabetes.

Food is turned into glucose — sugar — and released into the bloodstream. Per CDC, “When your blood sugar goes up, it signals your pancreas to release insulin. Insulin acts like a key to let the blood sugar into your body’s cells for use as energy.”

But the process doesn’t work the way it should when someone has diabetes. Either their body doesn’t make enough insulin or it can’t use it well. “When there isn’t enough insulin or cells stop responding to insulin, too much blood sugar stays in your bloodstream. Over time, that can cause serious health problems, such as heart diseasevision loss, and kidney disease,” CDC explains.

There are three types of diabetes.

In Type 1, diagnosed at any age, the body doesn’t make insulin and the person must take it every day to live. In Type 2, the body does a poor job of using insulin, so blood sugar levels fluctuate. Often, Type 2 can be delayed or prevented with lifestyle changes, such as a healthy diet, weight loss and more exercise. The third type develops in pregnant women and is thus called gestational diabetes. It usually leaves after the baby is born, but can create health problems for the child, and both mother and baby are at higher risk of Type 2 diabetes later.

Unproven claims

The FDA reported that smartwatches and smart rings marketed as measuring blood glucose “may be sold through online marketplaces or directly from the seller.” The agency tells people to instead talk to their health care provider about using an FDA-approved device that will meet their needs.

Consumers who may have gotten a bad reading from such a device are also being asked to report it to the FDA.

Dr. David Klonoff, of the Sutter Health Mills-Peninsula Medical Center in San Mateo, California, told CBS News that a number of companies have been working on devices to measure blood sugar without requiring piercing the skin. But none has created technology that’s accurate and secure enough to get coveted FDA approval. He added that trying to measure blood sugar in body fluids including tears, sweat and saliva have not panned out, either.

“It comes down to risk. If the FDA approves it, the risk is very small. If you use a product that is not cleared by the FDA, often the risk is very large,” Klonoff said.