The Food and Drug Administration is warning people about certain smartwatches and smart rings that claim to measure blood sugar without piercing the skin.

The warning doesn’t apply to smartwatch devices that show data from glucose monitoring devices that pierce the skin, the FDA said. Instead, it applies to any smartwatch or smart ring that is intended to measure or estimate blood glucose values on its own, none of which have been approved by the agency.

The devices, which have not been reviewed by the FDA, could provide inaccurate measurements of blood glucose levels, leading to serious health consequences.

“For people with diabetes, inaccurate blood glucose measurements can lead to errors in diabetes management, including taking the wrong dose of insulin, sulfonylureas, or other medications that can rapidly lower blood glucose. Taking too much of these medications can quickly lead to dangerously low glucose, leading to mental confusion, coma, or death within hours of the error,” the FDA said.

The smartwatches and rings in question, often sold online under multiple brand names, claim to measure blood glucose levels using non-invasive techniques but, in actuality do not directly test blood sugar. FDA is monitoring the sellers and “working to ensure (they) do not illegally market unauthorized smartwatches or smart rings.”