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, Cortez cautioned that unreliable and unregulated health apps could pose a significant threat.

"Besides wasting your money, these apps may actually do harm," 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."

More than 100,000 health apps are available in the iTunes and Google Play stores, according to Research2guidance, a mobile market research firm. By 2017, it estimates, the market for such tools, known as mHealth apps, will be $26 billion.

"Broadly defined, there could be hundreds of millions of mHealth users," Cortez said.

For some users, apps that help chart health are undoubtedly a good thing. Keith Wick, a 29-year-old information systems technician for the state of California, manages his Type 1 diabetes on his Android smartphone. Wick keeps track of his blood sugar levels by entering data six times a day onto a virtual spreadsheet.

"I’m on my phone all day, and I find it easier to keep track in a digital form," he said. "It’s much easier than keeping a tiny piece of paper with you, and folding it and refolding it into your pocket throughout the day."

Beyond serving as virtual notebooks, some apps work with external sensors. The AliveCor heart monitor, for example, incorporates a case, approved by the Food and Drug Administration, that fits around an iPhone to monitor basic heart rhythms.

The most reliable medical apps tend to result from collaborations among developers, physicians and experts in health law. HemMobile, a Pfizer app that helps hemophilia patients track their infusions, was supervised by a company review committee that consisted of a physician, a regulatory professional and a legal expert. Every aspect of the HemMobile program, from display to medical terminology, required the committee’s approval.

"It’s a very efficient system," said Bartholomew J. Tortella, a medical director on the hemophilia team at Pfizer. "We sign off on the idea, it gets built, it comes back, we test it, and we approve it."

But untested mHealth apps are occasionally presented as replacements for legitimate medical equipment. In some cases, the apps have drawn government scrutiny.

In 2011, the Federal Trade Commission fined a developer who claimed that his program, AcneApp, could treat acne with the light from an iPhone screen. Before it was removed from the iTunes store, almost 12,000 people had downloaded it. Last year, the FDA, which regulates medical devices, sent a letter to Biosense Technologies inquiring about its uChek app, which is designed to use the iPhone’s camera to interpret urinalysis strips. The app is no longer sold in the iTunes store for the United States.

"Patients rely on these apps when they should be seeking real medical advice," Cortez said. "Missed chances are a kind of harm."

Less rigorous health apps may include a disclaimer noting that the app is meant for entertainment purposes only. But fine print does not dissuade some consumers.

James P. Thompson, 68, a consulting social scientist from Farmington, Connecticut, paid $3.99 last summer for the Pulse Oximeter Heart and Oxygen Monitor app from digiDoc Technologies. Thompson has emphysema, and he hoped to use his iPhone to monitor his heart rate and blood oxygen levels.

"I don’t really understand the science behind how it’s done," he said. "I’m usually very curious, but about this I just accepted that it’s shining a light and somehow ascertaining oxygen saturation," as approved devices do.

In the iTunes Store, the Pulse Oximeter app description notes that it is not intended for clinical use. "We always say that people should not use it to monitor their diseases," said Damoun Nassehi, a physician and chief executive of digiDoc. "I don’t want them to rely on our app in any way."

Thompson was aware of the app’s disclaimer, but figured that it was worth a try. "It was one less thing to carry, and one less thing to lose," he said. But after the app’s blood oxygen readings differed sharply from those of his FDA-approved pulse oximetry device, he took a step back.

"I don’t think I would rely on it any further," he said. "I look at it and say, for four bucks, it’s not a bad deal."

In response to the proliferation of untested mobile health apps, the FDA has issued recommendations for developers and distributors. The guidelines note that the agency would enforce regulatory requirements only on apps that qualify as medical devices — anything used to diagnose, treat or prevent a medical condition. Bakul Patel, associate director of digital health for the FDA, said the agency would be unlikely to oversee a monitoring app like Pulse Oximeter.

But independent physician reviews may help consumers choose apps that meet standards for safety and efficacy. Dr. Iltifat Husain, founder and editor-in-chief of iMedicalApps, a review site, encourages people to read the entire app description before buying. And as ever, Husain says the best course of action is to speak with a doctor about apps that may aid treatment.

"Physicians are becoming more knowledgeable in this arena," said Husain. "Apps are going to be most effective when used in conjunction with your physician."

© 2015 The New York Times Company