But the FDA said the devices cannot be used to diagnose or rule out Covid-19, and generally it recommended not relying on them to assess health.
It recommended paying attention to other indicators of low oxygen, such as blueness of the face or a racing pulse.
The US health agency said the device might be “useful” for estimating blood oxygen levels, but multiple factors can affect the accuracy of a pulse oximeter reading, such as poor circulation, skin pigmentation, skin thickness, skin temperature, current tobacco use and use of fingernail polish.
The warnings come nearly two months after a study published in New England Journal of Medicine showed that the device was three times more likely to give misleading readings among African-American patients.
During the pandemic, the devices have also become a widely sold item online, used by people to monitor their own oxygen levels at home.
However, over-the-counter oximeters sold in stores or online do not undergo FDA review and are not intended for medical purposes, the agency said.