A bodybuilder presses the iPhone’s camera with a fingertip, and his heart rate and blood oxygen levels appear on the screen. A fellow in pajamas steps onto a scale, peers at his smartphone, and sighs dejectedly. A runner races along the waterfront, a cellphone strapped to her pumping arms.

Apple’s television commercials for the iPhone 5 portrayed the device as not just a smartphone, but a health and fitness tool. And indeed, iPhones, Androids and now even the Apple Watch provide countless applications to help with motivation and organization. But a subset of these apps go further, purporting to function as medical devices — to track blood pressure, treat acne, even test urine samples. Amid a proliferation of such apps, physicians and federal regulators are sounding an alarm, saying that programs claiming to diagnose or treat medical conditions may be unreliable and even dangerous.

“There’s just no plausible medical way that some of these apps could work,” said Nathan Cortez, an expert in medical technology law and regulation at Southern Methodist University’s law school in Dallas.

In an editorial in The New England Journal of Medicine last summer, Mr. Cortez cautioned that unreliable and unregulated health apps could pose a significant threat.

“Besides wasting your money, these apps may actually do harm,” Mr. Cortez said in an interview. “If you’re diabetic and your app is misreading your blood glucose levels, you may give yourself more insulin than you need and go into diabetic shock.”

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Jeffrey R. Ungvary President

Jeffrey R. Ungvary