Singh recently released an analysis of hundreds of health apps in which he concluded that many of them are functionally useless. But most of the apps criticized in that report were deemed useless because they did not provide sophisticated data analysis, a meaningful tracking function or communication with health care professionals, but not necessarily dangerous. 

What is worse, however, are apps that could lull their users into a false sense of security or, nearly as bad, render false diagnoses. For instance: A common app that measures blood pressure, but missed high blood pressure three-quarters of the time, or one that purports to diagnose skin cancer, but drastically overreacted to a mole on a seven-year-old girl.

Apps that claim to conduct the most serious medical services, and therefore pose the greatest risk by failing, may have to be reviewed by the Food and Drug Administration. But so far, regulation has not caught up to the technology.

"If things are really, really hurting people, we could … deal with them on a one-off basis as needed, but generally we would not take action," Bakul Patel, associate director for digital health at the FDA's Center for Devices and Radiological Health, told the LA Times.