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At a bachelor party two years ago in South Lake Tahoe, California, a tech worker who we'll call Owen glanced down at his Fitbit in between snorting lines of cocaine. He noticed his heart rake had spiked to 150, an abnormally high level considering he'd been sitting for hours. "My heart rate only gets to 150 if I'm running, like really intense physical activity," said Owen, who agreed to share his story on condition that we not use his real name. "If I'm in a really stressful work meeting, I might get close to 100 or 120." Owen had been indulging every 15 minutes or so, taking turns with his friends, as is customary with his group. Concerned about his elevated heart rate, he passed his Fitbit to someone who had just entered the room to see what would happen after his first line of cocaine. Sure enough, his friend's heart rate went from around 80 beforehand to 150 about 20 minutes later. "I think we all knew it would have an impact on our heart rate, but we'd never seen it happen before," Owen said. "It became interesting to keep an eye on." Cocaine can cause a user's heart rate to jump by unleashing dopamine into the body and producing a rush of adrenaline. An extreme increase in heart rate is far from the only risk posed by cocaine, which causes more than 5,000 people deaths a year from overdoses, according to the National Institute on Drug Abuse. Overdose deaths can be tied to heart attacks, strokes and angina, along with other complications, none of which can be prevented with an activity tracker. Cocaine is also highly addictive, and medical experts generally agree that even occasional use can be harmful to a person's health. Owen says he does what he can to keep himself safe. As a 20-something techie in San Francisco, he's submerged in a work-hard play-hard environment, where recreational drug use is an accepted norm by people who are simultaneously obsessed with data, devices and their health. Since that weekend in Tahoe, Owen said he brings his Fitbit along whenever and wherever he parties, whether it's at a night club, a dance party or Burning Man, the annual drug-fueled shindig in Nevada's Black Rock Desert.

The Man dominates the playa during the Burning Man 2015 "Carnival of Mirrors" arts and music festival in the Black Rock Desert of Nevada, August 31, 2015. Jim Urquhart | Reuters

It isn't likely to come up in casual face-to-face conversation, but scores of users on Reddit forums, Twitter and other social media sites write about the value of their Fitbit or Apple Watch in tracking their use of cocaine, ketamine, speed, and other drugs. Dozens of these threads have popped up in the past few years on the topic, some focused on cocaine and others on MDMA, also known as ecstasy. Representatives from Fitbit and Apple declined to comment for this story. Nick, a 23-year-old restaurant worker in Nevada, told CNBC that he got a Fitbit last Christmas. The morning after a recent night out, which involved both cocaine and alcohol, he checked the Fitbit app on his phone and saw his heart rate had surged to about 115 the previous night. Nick, who asked that we not use his last name or the city where he resides, said his resting rate is in the mid-80s. "I noticed that I was at a bar when the spikes happened," he said. "There wasn't anything that could have caused the heart rate increase except for the drugs." Nick had posted about his experience on Reddit. Another Reddit user, who goes by 3meopcpnumberonefan, posted on the website a screenshot from a health app, showing what happened after using cocaine. "Drugs are basically the only reason I wear a Fitbit," the post said. "I want an early warning system for when my heart's going to explode." There's even a YouTube channel called DrugsLab with more than half a million subscribers. Three hosts perform on-camera tests of drugs suggested by commentators, while their heart rate and body temperature are tracked on a board behind them. The idea, they say, is to promote drug education for millennials. But don't expect your doctor to condone the practice. Academics and medical professionals told CNBC that people who rely on a heart rate monitor to protect them from overdosing or from other ill effects of hardcore drugs are giving themselves a false sense of security.

'Always a risk'

Ethan Weiss, a cardiologist and associate professor at the University of California, San Francisco, said heart rhythm and blood pressure are also impacted by cocaine use and aren't currently trackable by most consumer smart devices. Even most heart rate monitors aren't foolproof. Many studies in recent years have found that popular heart rate trackers are less accurate than a standard chest strap. "Taking drugs is always a risk, whether you're monitoring a tracker or not," Weiss said. "It's possible this is leading people to do more cocaine." Owen, who does cocaine about six times a year, said his Fitbit helps him control his use, because when his heart rate gets uncomfortably high, he knows to skip a turn. He also said that, while he can't be certain of its accuracy, he knows that his Fitbit generally shows his resting heart rate at about 55 and his maximum exertion at around 180. So he has a baseline for comparison. "If someone says, 'Let's do a line,' I'll look at my watch," Owen said. "If I see I'm at 150 or 160, I'll say, 'I'm good.' That's totally fine. Nobody gives you a hard time."