The documentary suggests, however, that Tasers may be a cause of death. One example is Stanley Harlan, a 23-year-old man from Moberly, Missouri. Harlan was pulled over in 2008 for speeding in a 35 mph zone across from his house. As his mother watched from the front steps, responding officers tasered him for a total of 31 seconds (six times longer than police departments typically recommend, according to CBS). Harlan went into cardiac arrest. He died while his family looked on.

Tuttle emphasized that “both the medical examiner and the courts have found that TASER did not cause” Harlan’s death. However, Douglas Zipes, a leading expert in cardiac eletrophysiology, argues in the documentary that an electrical pulse from an electronic weapon can interfere with the normal heart rate and kill within 4 to 6 minutes. Zipes noted that the Stanley Harlan case is especially painful because of what happened after the initial shock. “I know he’s got ventricular fibralation and is dying,” he says, but the police, “don’t know that. They haven’t been taught to think of that. And this is a life that could have been saved.”

The documentary argues that the Stanley Harlan case highlights one of the bigger problems with Tasers—human error. While Tasers can and have been deployed safely (or at least, non-lethally), there are many variables in the field, including subjects who are intoxicated, may have underlying heart problems, or who may be shocked multiple times. We know Tasers can be non-lethal, but that doesn’t mean we should use them with the assumption that they are 100% safe every time.

A recent Guardian report found that 47 deaths resulted after police deployed Tasers in 2015—although the company TASER still aggressively disputes that its weapons caused these fatalities. The Guardian found that police department guidelines frequently fail to spell out best practices, for example not shocking a suspect multiple times, or to avoid shocking a suspect already in handcuffs. It also found that the majority of people who died this year after being tasered were “under the influence of alcohol or illicit drugs, or had psychiatric co-morbidities.”

Clearly, ECWs are less lethal than firearms—47 deaths pales in comparison to the number of Americans killed by police firearms, which numbers around 1,000 to date.”Most people who get tasered don’t sustain significant injuries, that’s absolutely true,” liability attorney John Burton says in the documentary. “It just hurts like a motherfucker.” So even if Tasers sometimes do result in death, the trade-off might be worth it if they reduced firearm usage, or if they de-escalated incidents in which more violent methods might have been used.

Speaking on behalf of TASER, Tuttle told Quartz that Tasers are “aren’t a replacement for deadly force ” In the documentary, the company argues that the weapons are not only are safe in themselves, but actually reduce harm. TASER CEO Rick Smith claims in the documentary that tasers have been used in more than 75,000 incidents, most of which prevented potential death and serious injury.

The problem with this argument, according to Killing Them Safely, is that Tasers don’t always seem to be used in situations where an officer would have otherwise used deadlier force. Stanley Harlan’s death occurred at a traffic stop. In video footage he seems confused, not belligerent. Another dashcam recording in the documentary from a 2007 traffic stop shows a man being tasered after what seems like a simple verbal altercation. Tasers, the documentary argues, are being used increasingly as a catch-all tool for officers, despite their potential consequences (not to mention physical suffering.)

TASER not surprisingly disputes this vigorously. “The field data has overwhelmingly demonstrated that Taser usage at the intermediate force level as a response-to-resistance is so effective that it often prevents many incidents from escalating to higher levels of force including situations where deadly force is necessary,” Tuttle told Quartz. But there is evidence that ECWs escalate violent altercations, rather than reducing them, is bolstered by research.

In an interview with Quartz, cardiologist and electrophysiologist Dr. Zian Tseng of the University of Califorinia San Francisco Medical Center pointed to his own 2009 study on the effects of Tasers. Tseng says he always looks at any type of public health intervention from the perspective of “does it cause overall benefit or harm?”

For his study, Tseng looked at sudden-death rates in 50 California police departments in the five years before and the five years after Tasers were introduced. What he found was that Tasers actually increased deaths. In fact, in the first year after Tasers are introduced, Tseng told Quartz, sudden death incidents increased by 600%. That number eventually dropped down to a 40% increase in death for the ensuing four years examined by Tseng.

“My interpretation,” Tseng says, “is [police] are told it’s a safe weapon and you don’t have to worry about brutality, and so they are looser with their policy. And then they realize they need to have more restrictive policies. That reduces the spike, but it’s still higher than before tasers were introduced.”

Tseng then investigated whether police shootings or officer injuries decreased after the introduction of Tasers. He found that neither had dropped. In fact, officer-involved shootings increased slightly. “The Taser actually escalates situations where you need to use a lethal firearm,” Tseng said. Tasers didn’t replace firearms; instead, they (slightly) increased their use.

All of which complicates the image of the Taser as an officer’s best friend. Brian Dolinar, an activist and writer who works with Champaign-Urbana Citizens for Peace and Justice, tells Quartz that in his experience, Tasers enable lazy policing and allow officers to not worry too much about actually deescalating the situation. Instead of reaching for their Taster, police should be working with family members and mental health providers, Dolinar said.

He added that Champaign-Urbana research notes a disparity in the demographics of those likely to be shocked. African-Americans are shocked the most. “It has a key racial component,” he said. “All the policing in the United States has a racial component, and tasers are no different.”

Killing Them Safely mostly focuses on the deaths attributed to Tasers. But it seems the bigger problem may be the way such ECW’s enable a problematic cycle of police force and escalation. Just because people survive the use of force doesn’t mean force was necessary.

So are Tasers are unsafe? Not necessarily. But police departments in particular need to be much more vigilant about their use. Killing people needlessly is a horrible injustice. Causing people needless pain is bad, too.

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